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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Fiberglass / Composites (boatbuilding) › Fiberglass & Composites Repair › Lesson 3
MFG-COMP18-06 · Lesson 3 · 18 clock hours · lab

Laminate Patch Repair: Wet Layup and Vacuum Bagging

Objective. Students will fabricate a multi-ply fiberglass laminate patch repair using both wet layup and vacuum-bagged methods, achieving a cured patch that is void-free on destructive cross-section inspection and meets the original laminate thickness within ±10%.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Laminate Patch Repair: Wet Layup and Vacuum Bagging

Why Repair Quality Matters

A structural laminate patch must restore the original stiffness, strength, and waterproofing of the hull or deck. Two defects doom a patch: voids (air pockets trapped between plies) and resin starvation (dry fiber). Both reduce load-carrying capacity and create pathways for water ingress. The goal of this lab is to produce a multi-ply patch that is void-free at a 10× cross-section inspection and falls within ±10% of the original laminate thickness.

Scarf Preparation Review

Before any wet work begins, the damaged area must be scarfed — tapered at a low angle into sound laminate. A scarf ratio of roughly 12:1 is a common industry starting point for structural repairs (check the applicable ABYC or manufacturer specification). The taper distributes load gradually back into the parent laminate. The repair surface must be dry, clean, and sanded to expose fresh fiber; residual gelcoat, wax, or contamination will prevent adhesion.

Wet Layup Patch Procedure

Surface prep: Wipe the scarf with a clean, lint-free cloth dampened with acetone. Allow full solvent flash-off before touching the surface — typically two to three minutes in normal shop conditions. Apply masking tape around the border; this acts as the release border and keeps the repair edge clean.

Resin mixing: Read the Technical Data Sheet (TDS) before every mix. Polyester resins use a catalyst (MEKP) dosed by percentage of resin weight, measured on a gram scale — never estimate by eye. Epoxy systems use a fixed mix ratio by weight or volume; use the ratio printed on the TDS. Incorrect ratios produce an under-cured, tacky, or brittle laminate. Be aware of pot life: once mixed, the resin begins to gel. In a warm shop, pot life shortens; mix only as much as you can wet out before the viscosity begins to rise noticeably.

Wet-out and ply sequence: Wet out each fabric ply on a flat board by stippling and squeegeeing resin into the cloth. The ply should be translucent, not white or dry. Lay the wetted ply into the scarf, working from the bottom of the scarf upward and from center outward to drive air toward the edges. Use a grooved aluminum roller — work in overlapping passes — to consolidate each ply and expel trapped air. Build plies to match the original laminate schedule (fiber type, weight, and orientation). Finish with a layer of peel ply: a woven release fabric that, when peeled off after cure, leaves a clean, slightly textured surface ready for bonding the next coat without re-sanding.

Vacuum Bag Repair Procedure

Vacuum bagging applies uniform atmospheric pressure across the laminate, compacting plies and drawing excess resin into bleeder cloth, which substantially reduces void content.

Bag stack (inside out from the laminate surface):

1. Wetted laminate plies

2. Peel ply

3. Perforated release film — controls resin flow into the bleeder; choose finer perforation to retain more resin

4. Bleeder/breather cloth — absorbs excess resin and distributes vacuum evenly

5. Bagging film sealed around the perimeter with sealant tape (tacky tape)

Leak check: Connect the vacuum pump. Watch the gauge: vacuum should pull to the target level specified on the resin TDS or instructor's direction, then hold steady. A slow pressure rise on the gauge means a leak. Run your hand around the sealant tape perimeter to find it; pinch or re-seat the tape. Do not proceed until the bag holds vacuum. Log vacuum readings every 15 minutes for the full cure cycle.

Why vacuum level matters: Too little vacuum leaves voids; too much can starve a low-viscosity epoxy of resin if the bleeder is too aggressive. Follow TDS guidance.

Cross-Section Inspection

After full cure, cut coupons across the repair with a diamond-blade wet saw. Wet cutting prevents heat build-up and keeps the cut face clean. Inspect the face under a 10× loupe: voids appear as white or dark pits. Measure thickness at three points with digital calipers. Compare to the original laminate schedule. Both repair types must meet the ±10% thickness criterion and show no voids larger than 1 mm to pass.

Safety

Key terms9

Scarf repairA repair technique in which damaged laminate is tapered at a low angle into sound surrounding material so that load is transferred gradually across the patch rather than at a sharp edge.
Pot lifeThe working time of a mixed resin system before viscosity increases to the point where wet-out and consolidation are no longer possible; controlled by temperature and batch size.
Peel plyA tightly woven release fabric applied as the final layer of a layup; when stripped after cure it leaves a clean, bondable surface without re-sanding.
Perforated release filmA thin plastic film with small holes placed between the peel ply and the bleeder cloth in a vacuum bag stack; it limits how much resin is drawn into the bleeder.
Bleeder/breather clothA porous, non-woven material placed above the release film in a vacuum bag; it absorbs excess resin and distributes vacuum pressure uniformly across the laminate surface.
VoidAn air pocket trapped between laminate plies; voids reduce mechanical strength, allow water ingress, and indicate insufficient consolidation or vacuum during cure.
Grooved aluminum rollerA metal roller with circumferential grooves used to consolidate wet plies and drive air bubbles toward the laminate edges during hand layup.
MEKP (methyl ethyl ketone peroxide)The peroxide catalyst used to initiate cure in polyester and vinyl ester resin systems; a strong oxidizer that must be measured accurately by weight or volume per the TDS.
Technical Data Sheet (TDS)A manufacturer document specifying mix ratios, pot life, cure temperature, vacuum target, and processing instructions for a specific resin system; must be consulted before every mix.

Worked examples4

1. You are mixing polyester resin for a wet-layup patch. The TDS specifies 1.5% MEKP by weight. You weigh out 200 g of resin. How many grams of MEKP do you add?
  1. Identify the resin mass: 200 g.
  2. Convert the percentage to a decimal: 1.5% = 0.015.
  3. Multiply: 200 g × 0.015 = 3.0 g of MEKP.
Answer: Add 3.0 g of MEKP to 200 g of resin.
2. Your original laminate measures 6.4 mm thick at three caliper points. After cure, your vacuum-bagged patch measures 5.9 mm at the same three points. Is the patch within the ±10% thickness tolerance?
  1. Calculate 10% of the original thickness: 6.4 mm × 0.10 = 0.64 mm.
  2. Determine the acceptable range: 6.4 mm − 0.64 mm = 5.76 mm (lower limit); 6.4 mm + 0.64 mm = 7.04 mm (upper limit).
  3. Compare the patch measurement to the range: 5.9 mm is between 5.76 mm and 7.04 mm.
Answer: Yes, 5.9 mm is within the ±10% tolerance band of 5.76 mm to 7.04 mm. The patch passes the thickness check.
3. Your vacuum bag is holding 22 in-Hg when you start the cure cycle. Fifteen minutes later the gauge reads 18 in-Hg with the pump still running. What does this indicate, and what do you do next?
  1. A dropping gauge reading while the pump runs indicates that air is entering the bag faster than the pump can remove it — there is a leak.
  2. Run your gloved hand slowly along the entire perimeter of the sealant tape to feel air movement; also check any fitting ports.
  3. When the leak is found, press the tape firmly back against the bag film or apply a small additional piece of sealant tape over the gap.
  4. Confirm the repair: the gauge should rise back to the target level and hold steady before restarting your 15-minute logging interval.
Answer: A pressure rise (vacuum drop) with the pump running means a bag leak. Locate the leak along the sealant tape or fittings, seal it, and verify the gauge returns to the target vacuum before continuing the cure cycle.
4. A patch repair requires 4 plies of 1708 biaxial fabric, each cut to 150 mm × 200 mm. You plan to wet out each ply with a 2:1 fiber-to-resin ratio by weight. A single ply of 1708 cut to that size weighs 42 g. How much total resin (in grams) do you need for all four plies?
  1. Find the total fiber weight: 4 plies × 42 g = 168 g of fabric.
  2. A 2:1 fiber-to-resin ratio means for every 2 parts fiber there is 1 part resin, so resin = fiber weight ÷ 2.
  3. Calculate resin needed: 168 g ÷ 2 = 84 g of resin.
Answer: You need 84 g of resin to wet out all four plies at a 2:1 fiber-to-resin ratio by weight.

The math in this lesson2

Learners compute MEKP grams at 1.5 percent and resin weight at a 2:1 fiber-to-resin ratio.
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Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates wet-layup patch sequence on a scarfed panel: wipe with solvent, apply release agent to tape border, mix polyester or epoxy resin per TDS (correct ratio, correct pot-life awareness), wet out each ply on a flat board, consolidate with a grooved aluminum roller, and apply peel ply as the final layer; students replicate the process on their own prepared panels. · Students set up a vacuum-bag repair on a second scarfed panel using perforated release film, bleeder/breather cloth, and bagging film; they connect a vacuum pump, check for leaks with a gauge (target vacuum level per instructor direction and resin TDS), and monitor bag for the full cure cycle, logging vacuum readings every 15 minutes. · After cure, students cut cross-section coupons from both repair types with a diamond-blade wet saw, inspect under a 10× loupe for voids and dry fiber, and measure thickness at three points with calipers; results are entered in the lab log and compared to the original laminate schedule.
Materials: Polyester, vinyl ester, and epoxy resin kits with hardeners/catalysts, woven roving, biaxial and chopped-strand mat (CSM), peel ply, perforated release film, bleeder/breather cloth, bagging film, sealant tape, vacuum pump, vacuum gauge, grooved aluminum rollers, plastic squeegees, mixing cups and sticks, gram scale, diamond-blade wet saw, digital calipers, 10× loupes, solvent (acetone), PPE as in Lesson 2, ACMA CCT reference manual
Check: Cross-section coupon shows no voids larger than 1 mm visible at 10× magnification; cured thickness is within ±10% of the target from the repair plan; lab log entry is complete with resin mix ratios, pot life, vacuum readings, and cure time—all three criteria must be met to pass.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Composites processing — surface preparation and secondary bonding
1. What is the primary purpose of a peel ply applied as the final layer of a wet layup patch?
Vacuum infusion and bagging — process monitoring
2. During a vacuum-bag cure, the gauge shows a steady drop in vacuum level even though the pump is still running. The most likely cause is:
Resin chemistry — mixing and catalyst ratios
3. A polyester resin TDS specifies 1.25% MEKP by weight. You measure 160 g of resin. How many grams of MEKP should you add?
Composites processing — vacuum bag stack assembly
4. Which layer in the vacuum bag stack directly controls how much excess resin is drawn away from the laminate into the bleeder cloth?
Quality control — laminate thickness inspection
5. A caliper measurement of a cured patch at three points gives readings of 7.1 mm, 7.3 mm, and 6.9 mm. The original laminate is 8.0 mm thick. The ±10% tolerance window is 7.2 mm to 8.8 mm. Which statement about these results is correct?
Workplace safety — hazardous materials handling
6. MEKP catalyst is classified as a strong oxidizer. Which safe-handling practice is most critical when working with MEKP and polyester resin in the same workspace?
Workplace safety — tool operation and dust control
7. When cutting cross-section coupons from a cured fiberglass laminate with a diamond-blade wet saw, the water feed must be kept running throughout the cut primarily to:
Composites processing — hand layup techniques
8. A grooved aluminum roller is used during wet layup primarily to:
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