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newSchoolWorks.com › VoTechWorks › Building & Construction › Wind Turbine Technician › Blade and Nacelle Composites Repair › Lesson 6
BLD-WIND70-06 · Lesson 6 · 18 clock hours · lab

Surface Preparation: Grinding, Scarfing, and Solvent Wipe

Objective. Students will prepare a damaged composite panel for structural repair by grinding a scarf bevel at the correct angle, cleaning with solvent per the repair manual, and achieving a surface profile verified by feeler gauge that meets the specified scarf ratio.

⚠ 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.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Surface Preparation Makes or Breaks a Composite Repair

A composite repair is only as strong as the bond between the patch and the parent laminate. No matter how carefully you cut your fabric or mix your resin, contamination or an incorrect scarf angle will cause the repair to fail — possibly catastrophically on a spinning blade. Surface preparation is therefore the most critical step in the entire repair sequence, and it must be performed exactly as the Original Equipment Manufacturer (OEM) repair manual states.

Understanding the Scarf Bevel

A scarf repair removes damaged laminate in a gradual taper rather than a blunt cut. This taper is described as a scarf ratio — the horizontal run of the bevel divided by the depth of the laminate removed. A common industry scarf ratio is 20:1 (20 mm of run for every 1 mm of depth), though the exact ratio is always specified in the OEM repair manual for the blade being repaired. A shallower taper spreads load over more area, giving the adhesive bond a larger surface and reducing stress concentration at the repair edge.

Think of it like splicing two boards at an angle: a steep, blunt joint is weak; a long, angled joint is strong. The same principle governs composite repairs.

Personal Protective Equipment Before You Touch the Grinder

Grinding composite laminates generates fine glass fibers and epoxy dust that are harmful to the lungs, eyes, and skin. Before any grinding begins:

Review the Safety Data Sheet (SDS) for both the composite material and any solvent before work begins. The SDS specifies permissible exposure limits, required PPE, and emergency procedures.

Grinding Technique: Coarse to Fine

Always start with a coarser grit (24-grit) to remove the bulk of damaged material quickly, then finish with a finer grit (60-grit) to smooth the taper and produce a surface the repair resin can bond to.

Body position matters. Stand to the side of the coupon so the guard is between you and the disc. Keep the grinder moving in consistent, overlapping passes — dwelling in one spot creates hot spots that can delaminate the surrounding undamaged laminate or thermally degrade the epoxy matrix. Work radially outward from the center of the defect area to maintain a consistent taper.

Verify your angle frequently using a protractor placed at the bevel edge. Check at multiple radial positions (the lab requires eight points) because hand grinding tends to produce a higher crown in some directions than others. Correct high spots with the fine-grit disc before moving to solvent wipe.

Measuring the Scarf with a Feeler Gauge

A feeler gauge set measures the step height between plies at any point around the bevel. By measuring the step height and knowing the ply thickness (from the repair manual), you can calculate the local run distance and confirm the scarf ratio is correct. You will also use the protractor to read the angle directly. Both methods cross-check each other.

Solvent Wipe Procedure

After grinding, the surface is covered with fine dust and skin oils from handling. Both will prevent proper adhesion. The solvent wipe must be performed exactly as the repair manual specifies:

1. Select the correct solvent — typically methyl ethyl ketone (MEK) or isopropyl alcohol (IPA), as listed in the repair manual and SDS. MEK is a more aggressive degreaser but has a lower flash point and requires careful handling. IPA is milder but effective for many systems.

2. Use lint-free wipes only — shop rags leave fibers embedded in the surface.

3. Wipe in one direction only, never back-and-forth. A back-and-forth motion drags contamination back onto the cleaned area.

4. Use a fresh wipe for each pass. Discard immediately.

5. Allow the solvent to flash off completely before applying any adhesive or resin — check the SDS for flash-off time at your working temperature.

Verify cleanliness with a white glove drag test or UV light inspection. Any visible residue means you must repeat the wipe.

Lockout/Tagout and Worksite Awareness

This lab uses electrically powered grinders. Before changing discs, ensure the grinder is unplugged (not just switched off). Follow your program's lockout/tagout procedure for portable electric tools. Keep cords clear of the grinding disc. Never reach across a spinning disc. Inspect the disc for cracks before mounting — a shattered disc is a serious projectile hazard.

Why Both Criteria Must Pass

The competency check requires both the angle criterion (six of eight points within ±2° of target) and the contamination criterion (clean glove wipe). These are not arbitrary — an incorrect scarf angle changes the bond area and load path; contamination prevents adhesion. A repair that passes one criterion but not the other is still a failed repair. If either criterion is missed, you will re-prepare a fresh coupon. This mirrors real field practice, where the technician, not the blade, pays for a failed repair.

Key terms10

Scarf ratioThe ratio of the horizontal run of a repair bevel to the depth of material removed; a higher ratio means a shallower, stronger taper (e.g., 20:1 means 20 mm of run per 1 mm of depth).
Scarf bevelThe tapered, angled surface ground into a composite laminate around a damage site to allow a patch to bond over a large area and transfer load gradually.
Feeler gaugeA set of precision metal blades of known thickness used to measure small gaps or step heights; used in composite repair to measure ply step height at the bevel edge.
P100 cartridgeA high-efficiency particulate air filter rated to remove at least 99.97% of airborne particles; required for composite grinding where fine glass and epoxy dust are generated.
Solvent wipeA cleaning step using a specified solvent (such as MEK or IPA) applied with a lint-free wipe in one direction only to remove grinding dust, oils, and contaminants before bonding.
Methyl ethyl ketone (MEK)A fast-evaporating solvent commonly specified for composite surface cleaning; effective at removing oils and epoxy residue but flammable and requiring adequate ventilation and PPE per the SDS.
Local exhaust ventilation (LEV)An engineering control that captures airborne dust and fumes at or near the point of generation; a vacuum shroud attached to a grinder is a form of LEV.
Hierarchy of controlsOSHA's ranked approach to hazard reduction: elimination, substitution, engineering controls, administrative controls, and PPE — with PPE as the last resort, not the first.
Flash-off timeThe time required for solvent to fully evaporate from a surface before the next process step; applying resin before flash-off is complete traps solvent and creates voids in the bond.
OEM repair manualThe Original Equipment Manufacturer's written instructions specifying authorized repair methods, materials, scarf ratios, and quality acceptance criteria for a specific blade design.

Worked examples4

1. A repair manual specifies a scarf ratio of 20:1. The laminate to be removed is 4 mm deep at the center of the damage. What is the minimum horizontal run (radius of the scarf bevel from the edge of the damage) required?
  1. Identify the scarf ratio: 20:1, meaning 20 mm of horizontal run for every 1 mm of depth removed.
  2. Identify the depth of material to be removed: 4 mm.
  3. Multiply: run = scarf ratio × depth = 20 × 4 mm = 80 mm.
  4. Confirm units: the answer is in millimeters, consistent with the inputs.
Answer: The scarf bevel must extend at least 80 mm horizontally from the edge of the full-depth damage area.
2. You use a feeler gauge to measure the step height between two adjacent plies at a point on the bevel. The step height reads 0.25 mm and the ply thickness listed in the repair manual is 0.25 mm (so exactly one ply is exposed at that step). The horizontal distance from that step to the next step outward is 5 mm. What is the local scarf ratio at that measurement point?
  1. The scarf ratio at any point equals the horizontal run divided by the depth change at that step.
  2. Horizontal run between steps: 5 mm.
  3. Depth change (one ply): 0.25 mm.
  4. Local scarf ratio = 5 mm ÷ 0.25 mm = 20.
  5. Express as a ratio: 20:1.
Answer: The local scarf ratio at that measurement point is 20:1, which meets a specification of 20:1.
3. You measure the bevel angle at eight radial points around a 75 mm diameter scarfed hole. The target angle specified in the repair manual is 3°. Your eight readings are: 3.5°, 2.8°, 3.1°, 4.5°, 2.9°, 3.0°, 3.2°, 2.7°. The acceptance criterion is that at least six of eight points must be within ±2° of the target (i.e., between 1° and 5°). Does this coupon pass?
  1. Determine the acceptable range: target 3° ± 2° = 1° to 5°.
  2. Check each reading against the range:
  3. 3.5° → within 1°–5°: PASS
  4. 2.8° → within 1°–5°: PASS
  5. 3.1° → within 1°–5°: PASS
  6. 4.5° → within 1°–5°: PASS
  7. 2.9° → within 1°–5°: PASS
  8. 3.0° → within 1°–5°: PASS
  9. 3.2° → within 1°–5°: PASS
  10. 2.7° → within 1°–5°: PASS
  11. Count passing points: 8 of 8 pass.
  12. Compare to minimum: 8 ≥ 6 required.
Answer: All eight points fall within the ±2° tolerance. The coupon passes the angle criterion with 8 of 8 points acceptable. (Note: the 4.5° reading is still within tolerance; it is only a concern if it trends toward the 5° limit, which the instructor should note for coaching purposes.)
4. A second student's eight bevel-angle readings are: 2.5°, 3.0°, 5.5°, 1.5°, 6.2°, 3.3°, 2.8°, 3.1°. Target is 3° ± 2° (acceptable range 1°–5°). Does this coupon pass or fail the angle criterion?
  1. Acceptable range: 1° to 5°.
  2. 2.5° → PASS
  3. 3.0° → PASS
  4. 5.5° → outside range (above 5°): FAIL
  5. 1.5° → PASS
  6. 6.2° → outside range (above 5°): FAIL
  7. 3.3° → PASS
  8. 2.8° → PASS
  9. 3.1° → PASS
  10. Count passing: 6 of 8 pass; 2 of 8 fail.
  11. Minimum required: at least 6 of 8. Exactly 6 pass.
Answer: The coupon just meets the minimum criterion of 6 of 8 points within tolerance and passes the angle check. However, two points at 5.5° and 6.2° are out of tolerance; the instructor should direct the student to identify and correct the technique causing the high spots before the next coupon.

The math in this lesson1

Learners apply a 20:1 scarf ratio to a 4 mm depth to get an 80 mm bevel run.
6.RP.3
In class: Grinding demonstration: instructor demonstrates angle-grinder setup, body position relative to the work surface, progression from coarse to fine grit, and measurement of scarf angle using a protractor; students observe and take written notes. · Scarfing practice: each student grinds a scarf repair bevel on a 300 mm × 300 mm glass/epoxy coupon with a seeded 75 mm diameter hole, targeting the scarf ratio specified in the provided OEM-style repair manual; instructor measures bevel profile at eight radial points with a feeler gauge. · Solvent wipe procedure: students wipe the scarfed surface with the specified solvent (methyl ethyl ketone or isopropyl alcohol per SDS and repair manual), using lint-free wipes in one direction only; instructor inspects with UV light or white glove check for contamination.
Materials: 4.5-inch angle grinder with 24- and 60-grit discs, vacuum shroud or local exhaust ventilation, hearing protection, disposable Tyvek suit, nitrile gloves, half-mask with P100 cartridge, 300 × 300 mm glass/epoxy coupons with seeded defects, protractor, feeler gauge set, solvent per SDS, lint-free wipes, OEM-style repair manual excerpt
Check: Instructor measures scarf bevel at eight points; at least six of eight points must be within ±2° of the specified angle, and the glove wipe must show no visible contamination; student retakes surface prep on a fresh coupon if either criterion fails.

Practice check8

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

NCCER Wind Turbine Tech — Composite Repair Fundamentals
1. What is the primary reason a scarf repair uses a gradual taper rather than a blunt-cut edge around the damage?
NCCER Wind Turbine Tech — Composite Repair Calculations
2. A repair manual specifies a scarf ratio of 15:1 and the laminate depth to be removed is 6 mm. What horizontal run is required for the scarf bevel?
GWO Basic Safety Training — Safe Work Practices; NCCER Wind Turbine Tech — Surface Preparation
3. When performing a solvent wipe on a scarfed composite surface, which technique is correct?
OSHA 10 — Personal Protective Equipment; GWO Basic Safety Training — Health Hazards
4. Which respiratory protection is required when grinding glass/epoxy composite laminates?
OSHA 10 — Hazard Recognition and Control
5. In OSHA's hierarchy of controls, where does PPE rank compared to engineering controls such as a vacuum shroud?
OSHA 10 — Lockout/Tagout; GWO Basic Safety Training — Electrical Safety
6. Before changing a grinding disc on a corded angle grinder, what must the technician do first?
NCCER Wind Turbine Tech — Quality Control and Measurement
7. A student measures the bevel angle at eight points. The target is 4° and the acceptance range is ±2° (2°–6°). The readings are: 3.5°, 4.0°, 7.1°, 4.2°, 3.8°, 4.1°, 1.5°, 3.9°. How many points are within the acceptable range, and does the coupon pass?
NCCER Wind Turbine Tech — Composite Repair Materials and Processes
8. Why must solvent flash off completely before repair resin or adhesive is applied to the scarfed surface?
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