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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Fiberglass / Composites (boatbuilding) › Fiberglass & Composites Repair › Lesson 4
MFG-COMP18-06 · Lesson 4 · 14 clock hours · lab

Core Replacement in Sandwich Panels

Objective. Students will remove deteriorated balsa or foam core from a pre-damaged sandwich panel section, install and bond new core material, restore the inner skin laminate, and verify bond quality by tap test before final inspection.

⚠ 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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Why Core Replacement Matters

Sandwich panel construction places a lightweight core material — end-grain balsa or closed-cell PVC foam — between two fiber-reinforced skins. The core provides stiffness and compressive strength without adding much weight. When water enters through a crack, fastener hole, or delamination, it saturates and softens balsa or degrades foam over time. A wet or rotted core loses nearly all its compressive strength and its ability to transfer loads between skins. Leaving deteriorated core in place is a structural and safety failure. Proper core replacement restores the panel to near-original structural properties when done correctly.

Identifying the Repair Perimeter

Before any cutting, probe and sound the damaged area with a tap hammer or a coin. A good sandwich panel returns a crisp, solid ring; a delaminated or wet zone returns a dull thud or hollow sound. Mark the perimeter at least 50–75 mm (roughly 2–3 inches) beyond the last hollow-sounding point. This margin ensures you remove all compromised material, not just the visibly damaged center. Use a felt-tip marker and a straightedge or compass to lay out a clean, geometric repair outline — rectangles and circles are easier to patch than irregular shapes.

Cutting and Removing the Outer Skin

Set a router with a depth stop or use an oscillating multi-tool to cut only through the outer skin. The single most common mistake is cutting too deep and damaging the inner skin. Measure the outer skin thickness with digital calipers at the panel edge or a spare coupon before you set your depth. Make your perimeter cut, then use chisels and scrapers to lift the outer skin flap. Work carefully — you may want to save the flap as a thickness reference.

Excavate all wet, soft, or discolored core with chisels and scrapers. Tap the inner skin as you go; it should remain solid. Discard all removed core material — never reuse it. If moisture is present in the cavity, dry the area thoroughly with a heat gun set to a low-to-medium temperature. Move the gun constantly; overheating can blister or post-cure the inner skin laminate prematurely and weaken it. The cavity must be dry and free of loose fibers before bonding.

Fitting Replacement Core

Cut replacement end-grain balsa or closed-cell PVC foam (such as Divinycell H-grade or equivalent) to fit the cavity using a template traced from the opening. End-grain balsa is used where compressive loads are high (decks, hull bottoms); closed-cell PVC foam resists moisture absorption and is common in high-curvature areas. Match the core thickness to the original using calipers — a mismatch will telegraph through the finished laminate and create a stress riser. Bevel or taper the core edges at roughly 12:1 if the repair plan calls for a tapered transition; check your specific repair plan before cutting.

Mixing and Applying Thickened Epoxy Adhesive

Mix epoxy resin and hardener according to the technical data sheet (TDS) for your system — ratio errors cause uncured or brittle bonds. Weigh components on a gram scale; do not estimate by volume unless the system is specifically designed for it. Add colloidal silica for gap-filling and non-sag properties, or a low-density filler (microballoons) where weight saving is critical. Mix to a peanut-butter consistency — the adhesive should hold a peak without slumping. Apply a wet coat of neat (unfilled) epoxy to the inner skin cavity first to promote adhesion, then trowel the thickened adhesive onto the inner skin and/or the back face of the core. Press the core firmly into place, ensuring full contact. Apply sandbag weights or vacuum pressure (if equipment is available) to maintain even contact during cure. Squeeze-out around the perimeter is normal and confirms adequate coverage.

Re-laminating the Outer Skin

After the core bond has fully cured (follow TDS cure times — do not rush), lay up replacement skin plies to match the original laminate schedule in the repair plan. Feather or scarf the edges of the existing skin at a minimum 12:1 ratio (length to thickness) so new plies overlap progressively onto sound laminate. Apply peel ply as the last layer to produce a bondable surface for any subsequent fairing. Allow full cure before tap testing.

Tap Testing and Inspection

Tap the entire repaired zone with a tap-testing mallet or a coin in a systematic grid pattern. A solid, consistent ring indicates good core-to-skin bond. A dull or hollow response indicates a void or delamination. Mark all suspect areas with a pencil. Minor edge voids can sometimes be injected with low-viscosity epoxy; large internal voids require re-opening and re-bonding. Your instructor will guide remediation before the panel is graded.

Safety Points

Key terms9

Sandwich panelA structural composite made of two fiber-reinforced skins bonded to a lightweight core material, combining high stiffness with low weight.
End-grain balsaBalsa wood cut perpendicular to the grain direction and used as core material; offers high compressive strength and good epoxy adhesion but absorbs water if the skin is breached.
Closed-cell PVC foamA synthetic foam core material (such as Divinycell) whose cells do not absorb water, making it resistant to moisture degradation in marine applications.
Thickened epoxy adhesiveEpoxy resin and hardener mixed with fillers such as colloidal silica or low-density microballoons to create a paste that fills gaps and resists sag during core bonding.
Tap testAn acoustic inspection method in which a mallet or coin is tapped across a cured laminate surface; a solid ring indicates good bond, while a dull or hollow sound indicates a void or delamination.
Depth stopA router accessory that limits bit travel to a preset depth, allowing the operator to cut through only the outer skin without damaging the inner skin.
Scarf (feather) ratioThe ratio of taper length to laminate thickness used when stepping or feathering new plies onto existing laminate; a 12:1 ratio is a common minimum for structural repairs.
Peel plyA woven release fabric applied as the final laminate layer; when peeled away after cure, it leaves a clean, textured surface ready for secondary bonding without additional abrasion.
Colloidal silicaA very fine fumed silica powder added to epoxy to increase viscosity and thixotropy, producing a gap-filling, non-sag adhesive paste.

Worked examples4

1. You measure the outer skin of a deck panel at 4.0 mm thick. Your router accepts depth settings in 0.5 mm increments. What depth should you set, and why might you choose to set it slightly less?
  1. Identify the target: you want to cut through the outer skin only — 4.0 mm.
  2. Select the nearest increment that does not exceed skin thickness: 4.0 mm is exactly on an increment, so 4.0 mm is theoretically correct.
  3. Consider router wobble, panel curvature, and measurement variability. Setting the depth at 3.5 mm reduces the risk of nicking the inner skin; any remaining skin material can be cut through manually with a chisel.
  4. Final setting: 3.5 mm, with the last fraction removed by hand tool.
Answer: Set the router depth to 3.5 mm — one increment below the measured skin thickness — to protect the inner skin, and finish the cut manually.
2. Your epoxy system requires a mix ratio of 3 parts resin to 1 part hardener by weight. You need approximately 120 grams of mixed adhesive. How many grams of resin and hardener should you weigh out?
  1. Total parts = 3 + 1 = 4 parts.
  2. Weight of one part = 120 g ÷ 4 = 30 g.
  3. Resin = 3 × 30 g = 90 g.
  4. Hardener = 1 × 30 g = 30 g.
  5. Verify: 90 g + 30 g = 120 g total. ✓
Answer: Weigh 90 grams of resin and 30 grams of hardener for a total of 120 grams of mixed epoxy adhesive.
3. A repaired deck panel measures 600 mm × 400 mm. After cure you tap-test the area in a 50 mm grid. Three adjacent cells in one corner return a hollow sound. What percentage of the total tested area is suspect, and what is the first remediation step?
  1. Each grid cell is 50 mm × 50 mm = 2,500 mm².
  2. Three cells: 3 × 2,500 mm² = 7,500 mm² suspect.
  3. Total panel area: 600 mm × 400 mm = 240,000 mm².
  4. Percentage suspect: (7,500 ÷ 240,000) × 100 = 3.125%, approximately 3.1%.
  5. First remediation step: mark the exact boundary of the hollow zone with a pencil, then consult the instructor to determine whether low-viscosity epoxy injection or re-opening is appropriate.
Answer: Approximately 3.1% of the panel area is suspect. Mark the boundary clearly and consult the instructor before proceeding with injection or re-opening.
4. The original laminate schedule calls for a skin thickness of 6 mm. Using a 12:1 scarf ratio, how long (in mm) must each new ply overlap onto the existing sound laminate at each edge of the repair?
  1. Scarf ratio = 12:1 means 12 mm of horizontal overlap for every 1 mm of laminate thickness.
  2. Skin thickness = 6 mm.
  3. Minimum overlap length = 12 × 6 mm = 72 mm.
  4. This 72 mm taper must be achieved on each side of the repair perimeter.
Answer: Each new ply must overlap at least 72 mm onto the existing sound laminate on each side of the repair perimeter.

The math in this lesson2

Learners split 120 g of epoxy at a 3:1 ratio and compute 12:1 scarf overlap for a 6 mm skin.
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Brush up in the math halls at newSchoolWorks.com:
In class: Instructor demonstrates the sequence for a deck core repair: marking the repair perimeter, cutting the outer skin with an oscillating tool or router set to skin depth, removing the outer skin flap, excavating and discarding all wet or rotted core with chisels and scrapers, drying the cavity with heat gun if needed, and fitting replacement end-grain balsa or closed-cell PVC foam with a template. · Students perform the full core removal and replacement sequence on assigned pre-damaged sandwich panels; they mix a thickened epoxy adhesive (with colloidal silica or low-density filler per TDS) to a peanut-butter consistency, bed the new core, apply weight or vacuum pressure, and allow full cure before re-laminating the skin. · After core bond cure, students lay up the inner skin plies to match the original schedule (confirmed from the repair plan), apply peel ply, and tap-test the entire repaired area after cure; defective spots are marked, and students discuss remediation steps with the instructor before the panel is submitted for grading.
Materials: Pre-damaged sandwich panels with balsa and foam core, oscillating multi-tool, router with depth stop, chisels, scrapers, heat gun, end-grain balsa sheets, closed-cell PVC foam (Divinycell or equivalent), epoxy adhesive kits, colloidal silica and low-density fillers, gram scale, mixing cups, brushes, peel ply, sandbag weights, vacuum equipment (optional), tap-testing mallets, digital calipers, PPE as in Lesson 2, ABYC composite repair reference
Check: Post-cure tap test of the entire repaired zone produces a solid ring throughout with no hollow areas; core-to-skin bond line is visible and continuous on at least one cross-section coupon taken from the panel edge—pass requires passing tap test AND visible continuous bond line.

Practice check8

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

Composite repair process and quality
1. When using a router to cut the outer skin of a sandwich panel for core removal, the most important reason to use a depth stop is to:
Quality inspection and defect assessment
2. During tap testing of a cured repair, a small area near the center of the patch returns a dull, hollow sound. The best immediate action is to:
Health and safety — PPE
3. Which respiratory protection is required when cutting or grinding cured fiberglass laminate?
Materials and mixing — composite adhesives
4. A technician mixes epoxy adhesive to a runny, water-like consistency and applies it to bond replacement core. What is the most likely problem with this approach?
Core materials and structural principles
5. End-grain balsa core is preferred over flat-grain balsa for deck sandwich panels primarily because end-grain orientation provides:
Surface preparation and repair process
6. After removing deteriorated core, the cavity feels slightly damp to the touch. Before bonding new core, you should:
Repair geometry and laminate schedule
7. The original laminate skin is 5 mm thick. Using a minimum 12:1 scarf ratio, the new ply overlap onto sound existing laminate on each side of the repair perimeter must be at least:
Laminate finishing and secondary bonding
8. Peel ply is applied as the final layer over a re-laminated repair skin. Its primary purpose is to:
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