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newSchoolWorks.com › VoTechWorks › Building & Construction › Wind Turbine Technician › Blade and Nacelle Composites Repair › Lesson 9
BLD-WIND70-06 · Lesson 9 · 16 clock hours · lab

Cosmetic Fairing and Gel Coat Finish Repair

Objective. Students will apply a polyester or vinyl ester gel coat filler to a structural repair, feather to the surrounding surface profile using board-sanding, and achieve a final surface finish within ±1 mm of the surrounding airfoil contour as measured by a contour gauge.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional engineer or master 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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Why Cosmetic Fairing Matters

A structural repair that is dimensionally correct but aerodynamically rough costs the turbine energy. Even small surface bumps or depressions on a blade's leading edge or pressure surface disturb the boundary layer, increase drag, and reduce annual energy production. The cosmetic fairing step restores the airfoil contour to within manufacturing tolerances after the structural laminate repair has fully cured. On a typical utility-scale blade, the accepted tolerance for surface deviation after repair is ±1 mm relative to the original profile. Achieving that tolerance requires careful mixing, controlled application, and disciplined sanding technique.

Materials Used in Gel Coat Fairing

Gel coat fillers used in blade repair are formulated from either polyester or vinyl ester resin. Vinyl ester fillers offer better moisture resistance and are preferred on blades that see high humidity or offshore environments, while polyester fillers are common for land-based applications. Both are supplied as two-part systems: resin and a peroxide catalyst (commonly methyl ethyl ketone peroxide, or MEKP). The manufacturer's data sheet specifies the correct mix ratio by weight or volume and the pot life at a given temperature. Deviating from the stated mix ratio causes incomplete cure, which leaves the filler soft, prone to print-through, and unable to hold a sanded edge.

Before mixing, verify the material is within its shelf-life date. Cold material cures slowly; material above roughly 35 °C (95 °F) may gel in the cup before application is complete. Mix in a clean, dry cup using a stir stick that reaches the bottom and sides of the cup, blending for the time stated on the data sheet.

Fill and Fair Technique

Apply the mixed gel coat filler with a flexible squeegee, spreading it in smooth passes parallel to the blade's chord. Overfill the repair area by approximately 1–2 mm proud of the surrounding surface. Overfilling is intentional: sanding removes the excess and leaves the final surface flush. A thin application that is underfilled will sand through before the profile is reached, requiring a second fill cycle.

Allow the filler to cure completely before sanding. A properly catalyzed filler will be firm, non-tacky, and will not dent under firm thumb pressure. Sanding partially cured material loads the paper with soft resin and produces an uneven surface.

Board Sanding with Progressive Grits

Board sanding is the key technique that produces a flat, contoured surface. A 300 mm rigid backing board distributes hand pressure evenly across the sanding paper, preventing the paper from dipping into low spots or bridging over high spots the way a soft sanding block would. Use long, overlapping strokes parallel to the chord direction.

Progress through grits in sequence: 80-grit removes the bulk of the overfill, 120-grit smooths the scratches left by 80-grit, and 220-grit prepares the surface for inspection and any topcoat. Never skip a grit step. Moving from 80 directly to 220 leaves deep scratches that are invisible until the surface is lit at a low angle but reappear when the blade is in service.

Keep the sanding board flat and maintain even pressure at both ends. If one end of the board rocks upward, a low spot will be sanded more aggressively than the high spots, and the repair will become a depression rather than a smooth blend.

Contour Checking

A flexible contour gauge is pressed against the undamaged blade surface adjacent to the repair to capture the original airfoil profile. The gauge is then moved over the repair and the gap (or overlap) between the gauge and the repaired surface is measured. Measure at five chord-wise stations distributed across the repair span. Mark any station where the deviation exceeds ±1 mm with a pencil, add filler or continue sanding as needed, and re-measure.

Always take the reference reading from sound, undamaged surface — not from another portion of the repair.

Final Inspection Under Raking Light

Raking light means holding a flashlight nearly parallel to the surface — typically at an angle of 10°–20° from the surface plane. At this angle, even very shallow surface defects cast visible shadows. Inspect for:

Photograph the finished surface under raking light and attach the images to the repair card before signing off.

Safety in the Fairing Lab

PPE: Wear nitrile or chemical-resistant gloves when handling gel coat resin and catalyst. MEKP is a strong oxidizer and skin sensitizer; contact causes burns and repeated exposure increases sensitization risk. Wear safety glasses or a face shield. A half-face respirator with organic-vapor cartridges is required when mixing, applying, or sanding cured composites in an enclosed space.

Ventilation: Work in a ventilated area. Styrene vapors from polyester resin are an irritant and are regulated under OSHA permissible exposure limits.

Sanding dust: Composite sanding dust contains glass fiber and cured resin particles. Wear a dust mask rated at least N95 when dry sanding, and a full respirator in confined or poorly ventilated areas.

Flammability: MEKP catalyst is flammable. Store it away from heat sources and dispose of waste catalyst according to the site's hazardous materials procedures.

Key terms8

Gel coat fillerA pigmented polyester or vinyl ester resin compound applied as the outermost surface layer of a blade repair to restore contour and protect the underlying laminate.
MEKP (methyl ethyl ketone peroxide)The peroxide catalyst blended with polyester or vinyl ester resin to initiate the chemical cure; it is a strong oxidizer and requires careful skin and eye protection.
OverfillIntentionally applying filler slightly proud (above) the surrounding surface so that the excess can be removed by sanding to achieve the correct final profile.
Board sandingA sanding technique using a rigid backing board to distribute pressure evenly, producing a flat, accurately profiled surface rather than one that follows hollows and high spots.
Contour gaugeA flexible measuring tool with pins or a flexible strip that conforms to a surface profile and can be transferred to the repair site to check dimensional accuracy.
Raking light inspectionHolding a light source nearly parallel to a surface (at a low angle) so that surface defects such as pinholes and scratches cast visible shadows and can be located.
Pot lifeThe working time after the catalyst is mixed into the resin before the material begins to gel and can no longer be applied; temperature strongly affects pot life.
Chord-wise stationA measurement location oriented along the chord (leading edge to trailing edge) of the blade, used to systematically sample the surface profile across a repair area.

Worked examples4

1. Your contour gauge reads 2.4 mm proud (high) of the surrounding surface at Station 3. How much material must board sanding remove to reach the center of the ±1 mm tolerance band (i.e., flush with the surrounding surface at 0 mm deviation)?
  1. Identify the current deviation: +2.4 mm (proud of surrounding surface).
  2. Identify the target: 0 mm deviation (flush with surrounding surface, which is the center of the ±1 mm pass band).
  3. Calculate material to remove: 2.4 mm − 0 mm = 2.4 mm.
Answer: Board sanding must remove approximately 2.4 mm of filler at Station 3 to bring the surface flush with the surrounding profile.
2. After the first sanding pass you re-measure five stations and record deviations (positive = proud, negative = low) of: S1 = +0.3 mm, S2 = −0.2 mm, S3 = +1.4 mm, S4 = +0.8 mm, S5 = −1.2 mm. Which stations pass the ±1 mm criterion, and which require further work?
  1. The pass criterion is −1 mm ≤ deviation ≤ +1 mm for every station.
  2. S1 = +0.3 mm → within ±1 mm → PASS.
  3. S2 = −0.2 mm → within ±1 mm → PASS.
  4. S3 = +1.4 mm → exceeds +1 mm → FAIL (sand more).
  5. S4 = +0.8 mm → within ±1 mm → PASS.
  6. S5 = −1.2 mm → exceeds −1 mm → FAIL (add filler, cure, re-sand).
Answer: Stations S1, S2, and S4 pass. Station S3 is too high by 0.4 mm and needs additional sanding. Station S5 is too low by 0.2 mm and needs a thin fill coat followed by cure and re-sanding.
3. A gel coat filler data sheet specifies a catalyst loading of 2% by weight. You have 150 g of resin in your mixing cup. How many grams of catalyst do you add?
  1. Convert the percentage to a decimal: 2% = 0.02.
  2. Multiply resin mass by the decimal: 150 g × 0.02 = 3.0 g.
  3. Verify units: grams of catalyst = 3.0 g.
Answer: Add 3.0 g of catalyst to 150 g of resin.
4. You are sanding with 80-grit and the repair area is 200 mm wide (chord-wise). Your rigid sanding board is 300 mm long. Describe why the board length relative to the repair width matters for achieving a flat result.
  1. A 300 mm board spans 200 mm of repair and 50 mm onto undamaged surface on each side.
  2. Because the board overlaps undamaged surface on both edges, it uses those sound surfaces as a reference datum, preventing the board from dipping into the repair zone.
  3. The undamaged surface stops the board from sanding deeper than the surrounding profile, acting as a natural depth stop.
  4. If the board were shorter than the repair, it would have no reference datum and could create a depression.
Answer: The 300 mm board spans beyond the 200 mm repair on both sides, so the undamaged blade surface acts as a reference datum and prevents over-sanding below the correct profile.

The math in this lesson1

Learners read a contour gauge showing +2.4 mm and compute filler to remove against a ±1 mm band.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Fill and fair: students mix gel coat filler, apply with a squeegee over the cured structural repair, overfill slightly, and allow to cure; after cure they board-sand with progressive grits (80, 120, 220) using a 300 mm rigid backing board to blend the repair into the surrounding surface. · Contour check: students use a flexible contour gauge at five chord-wise stations across the repair to measure the surface profile and compare readings to the undamaged blade surface adjacent to the repair; deviations are marked with a pencil and additional fill/sanding is applied. · Final inspection: students perform a visual inspection under raking light (a handheld flashlight at a low angle) to identify sanding scratches, pinholes, and color mismatch; they document findings with photographs and complete the cosmetic repair sign-off section of the repair card.
Materials: Gel coat filler (polyester or vinyl ester), mixing cups, squeegee, 80/120/220-grit sandpaper, 300 mm rigid sanding board, contour gauge, handheld flashlight, repair card, camera or tablet, color-match chart
Check: Contour gauge readings at all five stations within ±1 mm of surrounding surface = pass; visual inspection shows no pinholes >0.5 mm and no sanding scratches visible under raking light = pass; both criteria must pass.

Practice check8

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

NCCER Wind Turbine Tech – Composites Repair
1. Why is gel coat filler applied slightly proud (overfilled) of the surrounding blade surface?
NCCER Wind Turbine Tech – Composites Repair
2. A technician wants to move directly from 80-grit to 220-grit sandpaper to save time. What is the most likely consequence?
GWO Basic Safety Training / OSHA 10 – PPE
3. Which PPE item is specifically required when mixing or applying polyester gel coat filler in an enclosed work area?
NCCER Wind Turbine Tech – Quality and Inspection
4. When taking a contour gauge reference reading, from which surface should the measurement be taken?
OSHA 10 – Hazard Communication
5. MEKP catalyst must be handled with particular care because it is classified as which type of chemical hazard?
NCCER Wind Turbine Tech – Composites Repair
6. After the first board-sanding pass, contour gauge readings at Station 2 show the repair is 1.6 mm below the surrounding surface. What is the correct next action?
NCCER Wind Turbine Tech – Quality and Inspection
7. Raking light inspection is performed by holding a flashlight at what approximate angle relative to the blade surface?
NCCER Wind Turbine Tech – Composites Repair
8. A gel coat filler data sheet states a pot life of 20 minutes at 21 °C (70 °F). The lab temperature today is significantly higher than 21 °C. What should the technician anticipate?
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