ESI Composites 2 — Hand Layup, Patching & Gelcoat Repair

Before you start

How this course works

Composites 2 is the hands-on repair course and it is where most of the yard revenue actually lives. Almost everything here is a secondary bond onto old cured laminate, finished with a coating that has to disappear into what is already there. Two chapters are given entirely to feathering, because feathering is the single skill that separates a repair a customer can find from one they cannot. Fourteen chapters, a check on each, 80% on the final to certify.

Chapter 1

🔍Damage Assessment — Finding All of It

The chip is the size of a coin. The crazing around it runs six inches, and nobody looked.

The first job is not repair. It is diagnosis, and the failure mode that costs this yard money is always the same one: fixing the visible damage and leaving the rest.

A coin, a plastic mallet or the back of a screwdriver handle, tapped in a grid across the panel. Sound, well-bonded laminate rings sharp and bright. Delaminated or wet laminate goes dull and dead. Work outward from the damage until the sound comes back to sharp, and mark the boundary in pencil as you go. That pencil line, not the visible chip, is your repair area.

Fine cracks disappear on a dry gelcoat surface. Wipe the area with a clean solvent-damp rag and watch it flash off — cracks hold the solvent momentarily and stand out dark just before the surface dries. A light scuff with fine abrasive has the same effect: the cracks stay dark while the surrounding surface goes uniformly matt.

This decision sets the whole job, and it is not yours to fudge:

• Cosmetic — damage stops in the gelcoat. Gelcoat repair, feather, finish.
• Structural — fibers are cut, plies are separated, or the core is compromised. Laminate repair with a proper taper, then gelcoat over the top.
• Both — most real jobs.

Chapter 2

🛠Grinding Out — Removing the Damage, Not Working Around It

A crack gets filled without being opened. Six weeks later it is back, in exactly the same line.

Filler and gelcoat have no strength across a crack. If you fill over a crack without removing it, the crack is still there under your patch, the panel still moves, and it telegraphs straight back through. You cannot fill a crack. You have to remove it.

Every crack gets ground out to a vee that reaches sound material at the bottom and has open, accessible sides. The reason is mechanical: a vee gives the patch material a bonding area many times the width of the original crack, and it lets you see that you have reached the end of the damage.

• Gelcoat crack — vee out through the full depth of the gelcoat until you are on clean laminate at the base of the groove.
• Star crack — grind out the whole star, every leg, past the visible end of each crack. Legs always run further than they appear.
• Gouge into laminate — remove all damaged and delaminated material, then taper the laminate as covered in Chapter 6.
• Blister — open it, drain it, and let it dry properly. A blister ground out and patched while still wet will come straight back.

Ground surfaces are covered in dust that will act as a release agent. Vacuum thoroughly, then wipe with a clean rag and the solvent specified for that resin system, working in one direction and turning to a clean face of the rag frequently. A dirty rag redistributes contamination — it does not remove it. Let the solvent flash off completely before any resin goes on.

Chapter 3

🔬Hand Layup Technique — Wet-Out, Rolling and Consolidation

The laminate looks glossy and perfect. Under the gelcoat it is full of air.

Composites 1 Chapter 1 established that fiber content decides strength. Hand layup is the process where you personally are the resin metering system, so this chapter is about the physical habits that produce a good ratio and no air.

Wet-out is complete when resin has penetrated to every fiber and displaced the air between them — not when the surface looks wet. The visual cue is colour change: dry glass is white and opaque, and properly wet-out glass goes translucent and stays translucent.

• Wet the surface first, then lay the ply into it. Dropping dry fabric onto a dry surface and trying to wet through from the top traps air underneath.
• Work from the centre outward. Air has to have somewhere to go, and outward is the only direction that is not under more fabric.
• Add resin sparingly and work it. Most people add too much because it makes wet-out feel easier. It is easier — and it gives you a weaker, heavier laminate.
• Give it a moment. Fabric takes a little time to wick. A patch that looks dry often wets through on its own if you work the surrounding area and come back.

A grooved aluminium or plastic roller is not a paint roller and it is not there to spread resin. Its job is to squeeze air out sideways and consolidate the plies together. Work it in overlapping passes, and change direction — along, across, then diagonally. Air trapped between plies is a void, and a void in a repair is a place the repair can start to come apart.

1. Cut and label every ply before you mix anything.
2. Stage brushes, rollers, gloves, resin, catalyst, scales, timer and waste container in the bay.
3. Mix to a batch size you can actually apply inside the working time, not to the size of the bucket.
4. Start the timer when the catalyst goes in. Know your gel time, do not estimate it mid-job.
5. On anything large, plan the batches and the sequence before the first one is mixed.

Chapter 4

📏Feathering 1 — The Geometry and Where to Start

The patch is perfect. You can still see a ring around it in low sun from thirty feet.

Feathering is turning a step into a slope. Any repair creates an edge where new material meets old. The human eye cannot see a slope, but it sees a step from across a marina — because a step catches light along a hard line. Everything in this chapter and the next exists to eliminate that line.

A structural repair needs both, and the cosmetic feather is always wider than the structural taper — it starts outside it and runs further out.

Think about what you are actually building. Gelcoat is a coloured, opaque layer of a certain thickness. Where you have sanded it thin, its colour shifts slightly and the substrate influences it. If that transition happens over a short distance, you get a visible halo or witness ring — a circle of slightly different colour and gloss around your patch.

Spread the same transition over a long distance and the change per inch becomes too small for the eye to catch. Nothing else changed. Only the slope.

The starting point is not arbitrary and it is not chosen by the size of your sanding disc.

1. Establish the sound boundary first. From Chapter 1 — the tap-test line, plus the true end of every crack. Nothing inside that line is a starting point.
2. Start the feather outside all damage, on 100% sound, well-bonded coating. If you start the feather on material that is itself cracked or debonded, the whole feather will lift later.
3. Work inward. The feather is cut from the good material toward the repair, not from the repair outward. That is what controls the depth at every point.

The governing principle is the thicker the coating you are cutting through, the wider the feather must be. Two practical rules used in this yard:

Almost nothing on a boat is flat. A rigid pad on a curved surface cuts the high spots and misses the hollows, which produces exactly the flat spot that shows under gloss. Use a soft interface pad, keep the tool flat to the local surface, and keep it moving in overlapping passes. On tight curves and in corners, go to hand sanding on a flexible block — a machine will always cut a flat there.

Chapter 5

✅Feathering 2 — Grit Progression and Where to Stop

"It looked perfect matt. Then we polished it and there was the ring."

Chapter 4 was the shape. This is the surface. The rule that governs everything here: every grit must completely remove the scratch pattern of the grit before it. If it does not, that coarser scratch is still in the surface and it will appear the moment you put gloss on it.

Grit steps are a working standard for gelcoat. Paint systems follow the coating manufacturer schedule — see Chapter 13 and the product data sheet.

Sand each grit at roughly right angles to the one before, or in a clearly different pattern. When the new grit has erased every trace of the previous direction, that grit is finished. This is the simplest reliable way to see that you have fully removed the last scratch pattern rather than assuming it.

• Keep the paper and surface genuinely wet. A dry patch of wet-and-dry loads up and puts a deep scratch straight through your work.
• Rinse the surface and the paper often — a single trapped coarse particle will draw a line across everything you have done.
• Use a block or a soft pad. Fingers put pressure through in ridges and you will sand grooves that match your fingertips.
• Wipe and inspect frequently. Wet sanding hides its own progress until you dry it.

Chapter 6

📐Structural Laminate Repair — The Scarf and the Ply Schedule

The repair is butted flush against the old laminate. It has almost no strength across the joint.

A laminate carries load along its fibers. If you cut a hole and fill it with new laminate that simply butts against the old, the only thing joining them is resin in shear across a knife edge. That joint is a fraction of the strength of the panel around it.

The answer is a scarf: grind the edge of the hole back to a long shallow taper so that each ply of the old laminate is exposed as a band, and each new ply can lie on top of it over a large area. Load transfers gradually from old fibers to new instead of jumping across a joint.

The taper is specified as a ratio of taper length to laminate thickness. A 12:1 scarf means the taper runs 12 units of length for every 1 unit of laminate thickness — so a laminate half an inch thick needs a taper six inches long on every side of the hole.

12:1 is the ratio given in the WEST SYSTEM fiberglass boat repair guidance and it is the ESI default for structural laminate repair. Some specifications call for a shallower taper still.

12:1 is quoted from WEST SYSTEM fiberglass boat repair guidance. Confirm the required scarf ratio against the applicable repair specification or manufacturer procedure for the vessel before use.

1. Mark the taper boundary before you start — thickness × 12, measured out from the edge of sound material all the way round.
2. Cut with a random orbital and a soft pad, 36 to 80 grit, working from the outside boundary inward.
3. Read the plies as you go. A correctly cut scarf shows the laminate as a series of concentric bands, one per ply. Those bands are your proof the taper is even — if a band is wide in one place and narrow in another, the taper is uneven there.
4. Stop at sound, well-bonded material at the outer edge, feather to nothing, and check with a straightedge that there is no step anywhere in the taper.

The repair replaces what was removed. That means matching, as closely as the job allows:

• Number of plies — count them in the scarf bands.
• Weight and type of each ply.
• Orientation of each ply. A biaxial replaced with a 0/90 in the same position is not the same laminate.
• Sequence — rebuild in the same order.

Plies are cut so that the smallest ply goes in first, at the bottom of the scarf, and each subsequent ply is larger, following the taper outward. The final ply overlaps onto the original surface beyond the scarf. Built this way, every ply lands on a fresh band of prepared laminate and the joint is staggered instead of stacked.

• Wet the prepared scarf surface with resin first, then lay the first ply into it.
• Work air out from the centre toward the edges with a grooved roller. Air trapped in a repair is a void in the joint.
• Watch for bridging into the corner at the bottom of the scarf — press the ply into the corner, do not let it tent.
• Do not exceed the working time. Batch the resin if the repair is large.
• Vacuum bagging a repair, where practical, consolidates the plies and improves the ratio. That is Composites 4.

Chapter 7

🔗Secondary Bonding — Where Repairs Actually Fail

The laminate is perfect. It peels off the hull in one clean sheet six months later.

A primary bond is formed when new resin cures against resin that has not yet fully cured — the two cross-link into each other chemically. That is what happens between plies laid up in one wet session, and it is very strong.

A secondary bond is new resin onto fully cured laminate. There is far less chemical cross-linking available, so the bond depends heavily on adhesion to a mechanically prepared, clean surface. Essentially every repair you will ever do at ESI is a secondary bond, and it is the plane where repairs fail.

1. Remove everything that is not laminate. Wax, polish, release agent, antifouling, old coatings, grease, salt. Wash and degrease before you sand — sanding a waxed surface grinds the wax into the laminate instead of removing it.
2. Abrade to fresh material. The whole bond area, uniformly, with no glossy patches left anywhere. A shiny spot is an unbonded spot.
3. Remove the dust. Vacuum, then a clean solvent wipe with a rag turned frequently to a clean face.
4. Let it flash off completely, then bond promptly. A prepared surface starts collecting dust, moisture and handling contamination the moment you walk away from it.

This is the chapter where the resin choice from Composites 1 pays off. Polyester has the weakest adhesion to cured laminate of the three systems, and a secondary bond is precisely the joint where that weakness shows. Vinyl ester bonds substantially better to prepared cured laminate, cures fast enough to fit a working day, and remains compatible with polyester gelcoat over the top. Epoxy adheres better still — but you cannot finish it with polyester gelcoat, so it is used where the specification calls for it or where you are bonding to a dissimilar material.

Chapter 8

🧱Core Repair in a Sandwich Panel

The soft spot is a foot across. The wet core runs four feet under sound-looking skin.

Composites 1 Chapter 6 covered why a sandwich panel works: the core holds the skins apart and carries shear. A core repair is not a filling job — it is restoring that separation and that shear path.

Wet and failed core always runs further than the soft area. Tap test in a grid outward in every direction until the ring comes back sharp, and mark it. Moisture meter readings help but are influenced by skin thickness and surface condition — use them alongside tapping, not instead of it. On a deck, check around every fitting and fastener in the area: that is nearly always the water entry point.

1. Cut the skin back to the marked boundary. Cut only the skin — set the depth so you do not cut into the opposite skin.
2. Remove all failed and wet core. Not most of it. If it is damp, it goes.
3. Dry the cavity thoroughly. Time, warmth and air movement. Closing up a damp core guarantees the repair fails, and it will fail as a bond failure across a wide area, not a small spot.
4. Cut and fit new core to match the original thickness and type. Chamfer the perimeter of the existing core where the new core meets it, so the skin does not bridge.
5. Bond the core in with full contact. Thickened resin, adequate coverage, and pressure applied while it cures — vacuum bagging, weights, or clamping through a caul. A core dropped onto a thin smear of resin has no shear path.
6. Rebuild the skin to the original schedule over a proper scarf (Chapter 6).
7. Refinish — Chapters 9 to 12.

Chapter 9

🎨Gelcoat Patching — Materials and Application

The patch is still tacky the next morning. Nobody put anything over it.

Gelcoat patching is not painting. Patch gelcoat is a thick, catalysed, pigmented resin that you are building up as a solid layer and then cutting back to the surrounding surface.

Polyester gelcoat is air-inhibited. Oxygen at the surface stops the cure, so gelcoat exposed to air stays permanently tacky no matter how long you leave it. In a mould this never matters because the laminate covers it immediately. On an open repair patch it matters completely. Three ways to defeat it:

• Use the manufacturer catalyst percentage for that product at that temperature. Measure it. Composites 1 Chapter 4 applies here in full.
• Too little catalyst gives a soft patch that never fully hardens and will not sand cleanly — it gums the paper.
• Too much catalyst gives a short working time, more shrinkage, more heat, and can yellow the patch — which ruins your colour match.
• Mix small. A gelcoat patch uses very little material and a big pot is wasted material and a fire risk.

1. Surface is prepped, feathered (Chapters 4 and 5), clean, dry and solvent-flashed.
2. Force the first material firmly into the bottom of the vee so there is no trapped air at the base of the repair. Air at the bottom of a patch becomes a pinhole later.
3. Build the patch proud of the surrounding surface — deliberately higher than finished level. Gelcoat shrinks as it cures, and you need material to cut back. A patch applied flush finishes hollow.
4. Seal the surface: wax additive, PVA, or film pressed down from the centre outward to squeeze air out.
5. Let it cure fully. Do not start sanding a patch that is still soft — it will gum, tear and drag out of the repair.

Chapter 10

🌈Colour Matching and Tinting

It matched perfectly in the shop. On the boat in daylight it is a different colour.

Colour matching is the step where good repairs get judged, and it is the step most often approached by guesswork. Two ideas make it manageable.

The hull in front of you has been in UV, salt and weather for years. It has chalked, faded and shifted. Gelcoat mixed to the original factory code will not match it — it will match what the boat looked like when it was new, which is exactly the wrong target.

So the first move on any colour match is: compound and polish a small area of the surrounding surface first. That does two things. It shows you the true current colour under the chalk, and it is what the area will look like when you finish the job anyway. Matching to a dull, oxidised surface that you are about to polish is matching to something that will not exist by the end of the day.

• Test on the boat, not on the bench. Put a small test patch on or immediately beside the repair area.
• Cure it, then judge it. Wet gelcoat is not the colour it will be cured. Do not decide from a wet sample.
• Judge in daylight, and from several angles. Shop lighting hides differences that daylight exposes instantly. Metamerism is real: two colours can match under one light and not another.
• Judge from the distance the customer will look from, and then from close up. Both matter.

1. Start from the closest available base. Tinting a long way from the base never ends well.
2. Add pigment in very small increments and mix thoroughly between each. Pigment is powerful and you cannot take it back out.
3. Write down every addition. Base, pigment, quantity, in order. This is the single highest-value habit in colour matching: it lets you repeat the mix for a second patch, and it builds a yard record for that hull.
4. Mix more than you need. Re-matching a second batch to a patch you already applied is far harder than mixing extra once.

Chapter 11

✨Cut, Compound and Polish

It is perfect. Two weeks later there are swirl marks all over it in the sun.

This is the same principle as the grit progression, continued with finer and finer abrasives. Compound is abrasive suspended in a carrier. Cutting compound has coarse abrasive, finishing polish has fine. Each step must fully remove the marks of the step before it.

A rotary polisher generates real heat, and gelcoat and paint will burn. Burn-through is not repairable by polishing — you are back to patching.

• Keep the machine moving in overlapping passes. Never let it dwell.
• Let the pad do the work. Pressure generates heat without improving cut.
• Work small areas and check often. Touch the surface — if it is hot, stop.
• Watch the edges. Corners, chines, mouldings and any high spot have thin coating and no heat sink. Almost all burn-throughs happen on an edge.
• Keep pads clean. A loaded pad cuts unevenly and holds grit that scratches.

Swirl marks are fine circular scratches from compound or a dirty pad, visible in direct sun. Holograms are the shimmering three-dimensional pattern a rotary leaves in a dark finish. Both come from the same source: a step was not fully removed by the next one.

You have just polished a patch to a high gloss in the middle of a hull that has not been polished in three years. The repair now stands out because it is better than everything around it. That is still a visible repair.

The right answer is usually to polish out to a natural break — a rubrail, a chine, a moulded feature, a boot stripe, a taped line. The eye compares across a continuous surface, and a break gives it nowhere to compare. Where there is no break, polish the whole panel and quote it that way.

Chapter 12

🪤Textured and Nonskid Deck Repair — Taking a Pattern Off the Boat

The patch is structurally perfect and glass smooth in the middle of a diamond nonskid deck.

A moulded nonskid pattern was made in the builder mould and it does not exist anywhere else. When you patch through it, you have to recreate it — and the only perfect source for that pattern is the undamaged deck a few feet away.

• Same pattern, same orientation. Many decks run the pattern directionally. Note the direction and mark it on the mould as you make it.
• Undamaged, unworn and clean. A worn traffic area gives you a worn pattern.
• Large enough to cover the repair with generous margin all round. You need spare pattern to blend the seam.
• As flat as possible. Taking a pattern off a heavily curved area and applying it to a differently curved area will not sit down.

Here is the critical point, and it is the one that damages customer decks when people get it wrong. A moulded nonskid texture is full of small undercuts. A rigid mould cast onto it will key into those undercuts and lock on. Removing a rigid mould from a textured deck risks pulling gelcoat off the boat.

Sequence:

1. Clean the donor area thoroughly — wash, degrease, and get every trace of dirt out of the texture. Whatever is in the pattern gets recorded in the mould.
2. Mask a generous boundary and build a simple dam so the silicone stays where you want it.
3. Apply release appropriate to the material you are casting and to a finished gelcoat surface. Follow the product instructions — and prove it on a test area first if you have any doubt.
4. Cast the silicone. Work it into the texture so no air is trapped in the pattern — every bubble is a missing feature in your reproduction. Build enough thickness to be handled without distorting.
5. Add a backing if the mould needs to hold its shape — a rigid backer bonded to the back of the silicone while it is still on the deck, so it records the local curvature.
6. Mark the orientation arrow on the back of the mould before you remove it. You will not be able to work it out afterwards.
7. Peel it, do not lever it. Start at one corner and peel back steadily, letting the silicone flex out of the undercuts. If it resists, stop and work the edge free rather than forcing it.
8. Clean the donor area and confirm it is undamaged before you move on. Check it in raking light.

1. Complete the structural repair and bring the patch area up to just below finished deck level. The pattern gelcoat is the last layer and it needs somewhere to sit.
2. Prep and feather the surrounding area as normal (Chapters 4 and 5) — but understand the feather here has to disappear into a texture, which is far more forgiving than a gloss surface. Use that.
3. Treat the pattern mould with release so it will let go cleanly.
4. Apply the colour-matched gelcoat generously to the patch area.
5. Press the mould in from the centre outward, squeezing air out to the edges. Air trapped under the mould is a flat spot in the pattern.
6. Hold it down under even pressure while it cures. The mould face excludes air, so air inhibition takes care of itself — the same principle as the Mylar method in Chapter 9.
7. Peel the mould off when fully cured, again peeling rather than levering.
8. Blend the seam. This is the step that decides the job. The edge of the mould leaves a line where new pattern meets old. Work that boundary by hand — carefully knock back any proud edge, and match the transition so the pattern runs continuously. Do not machine sand into a texture; you will flatten it.

A pattern mould taken off a boat is an asset. Label it with the vessel, the make and model, the pattern and the orientation, and store it flat. The next time that model comes into the yard — and it will — the hardest part of the job is already done.

Chapter 13

🖌️Repairing Painted Surfaces — Awlgrip, Awlcraft and Blending

A scuff on an Awlgrip topside gets buffed. Now there is a dull patch instead of a scuff.

Composites 1 Chapter 5 set up the distinction. This is where you use it.

A paint job is a system: substrate, fairing, primer, topcoat. A repair rebuilds whatever layers the damage went through, in order:

1. Substrate — if the damage reaches laminate, that is a Chapter 6 structural repair first. Paint over a bad laminate repair is still a bad repair.
2. Fairing — restore shape. On a painted surface the shape is what the gloss shows, and any unfairness is magnified by the finish.
3. Primer — Awlgrip 545 or whatever the system specifies, applied and sanded per the data sheet. Feathered into the surrounding primer or prepared topcoat.
4. Topcoat — sprayed, to the manufacturer film build, number of coats and recoat windows.

Spraying a hard edge of new topcoat onto old topcoat gives you a visible line, every time. Two ways round it:

Where there is neither a blend nor a break available, the honest answer is that the panel gets recoated, and that goes on the estimate.

• Degrease before sanding, same as Chapter 7. Sanding a contaminated surface drives contamination in.
• Sand to the grit the data sheet specifies, not the grit you happen to have. Too coarse and the scratch telegraphs through the gloss; too fine and the coating has nothing to grip.
• Tack off immediately before spraying, and control the air. Dust that lands in wet paint is in the finish permanently.
• No silicone anywhere. Composites 1 Chapter 8. A trace of silicone gives fisheyes across the whole sprayed area.

Chapter 14

✅Quality Control, Documentation and Kaizen

The job is signed off. Two months later nobody can say what resin went into that hull.

Repair is the part of the yard where quality is hardest to inspect, because everything that matters is buried under the finish. Nobody can see your bond prep, your ply schedule or your scarf once the gelcoat is on. That is exactly why the discipline and the record have to be right.

• Photos: before, boundary marked, ground out, laminate in, finished.
• The assessment — what you found and how far it ran.
• Materials: resin system, reinforcement type and weight, ply count, orientation, core type.
• The colour formula, keyed to the vessel.
• The coating system on that hull, if you established it.
• What slowed you down, your trip count, and any ** flags with the manager tagged.

The cheapest place to fix a defect is the station where it was made. In repair that is unusually true, because every subsequent step buries the last one. A dry spot passed on to the finisher becomes a blister under fresh gelcoat. A short feather passed on to the polisher becomes a halo that cannot be polished out.

Final certification

🎓Final exam

Thirty-four questions drawn from all fourteen chapters. You need 80% to pass. Put your name on the certificate and your completion is recorded to ESI training automatically.

ESSENTIAL SHIPYARD UNIVERSITY

Certificate of Completion

This certifies that

has successfully completed ESI Composites 2 — Hand layup, patching, gelcoat repair, feathering & nonskid — and passed the final examination to the ESI standard.

Jason Knott

President

Essential Shipyard Industries

★CERTIFIED

Date Issued

Essential Shipyard University