Material guide
Fiberglass
What works on fiberglass and gelcoat: drilling without star cracks, the blade that cuts it, the right repair epoxy, and how to handle the dust.
Quick facts
- What it is
- Glass fibers set in hard resin, often with a glossy gelcoat skin on the show side
- Best drill bit
- A sharp twist bit, then a countersink to chamfer the gelcoat edge so it won't crack
- Best blade
- Diamond-grit reciprocating blade, rated for fiberglass
- Best repair glue
- Epoxy. Clean off wax, then sand to a dull finish with 80-grit.
- Fastening
- Bond fasteners in epoxy. For small ones, drill the hole about twice the shaft size and fill it.
- Dust and fumes
- Glass fibers are hard for the lungs to clear. Polyester resin gives off styrene.
Fiberglass is glass fibers locked in a hard resin. It's light, strong and doesn't rot, so it's used for boats, tubs, shower pans, hot tub shells and roof panels. The show side often has a gelcoat, a thick glossy resin skin.
That makes fiberglass two materials in one. The glass fibers are abrasive, so they dull toothed blades fast. The gelcoat is hard and brittle, so it chips and cracks around holes if you're rough with it.
The tricks follow from that. Use abrasive blades, like diamond grit, that grind instead of tear. Go easy with drill bits and chamfer every hole in gelcoat. For repairs, use epoxy on a clean, sanded surface.
Fiberglass dust is the other big issue. Tiny glass fibers make your skin itch and irritate your eyes and lungs. Wet methods, good airflow and a mask make a big difference.
Drill into fiberglass
Tape over the spot and mark it. Drill with a sharp bit and light pressure, and back the panel if you can. Then chamfer the gelcoat edge with a countersink. Do the same before a screw goes in, and don't overtighten, or the gelcoat cracks.
What to use: Sharp HSS bit + countersink
Guides
- How to drill fiberglass and gelcoat (the reverse-start trick) (coming)
- How to countersink a screw (and how deep) (coming)
Cut fiberglass
For tubs, panels and hot tub shells, use a reciprocating saw with a diamond-grit blade. It lasts far longer than a toothed blade in glass fibers. Tape the cut line, support both sides and wear a respirator.
What to use: Diamond-grit recip blade
Guides
- What blade to cut up a hot tub (and how to do it safely)
- How to cut fiberglass (tubs, panels, boats) and what blade to use (coming)
- Reciprocating saw blade guide (TPI for wood, metal, pruning) (coming)
Glue fiberglass
Repair cracked fiberglass with epoxy. Wash off wax and silicone first, then sand the area to a dull finish with 80-grit paper. Epoxy grips cured polyester fiberglass well. It also bonds a patch of glass cloth into the repair.
What to use: Marine epoxy + glass cloth
Guides
- Repair adhesive for cracked fiberglass (tub, panel, boat) (coming)
- Will new epoxy stick to cured epoxy? (coming)
- Construction adhesive vs epoxy vs polyurethane (coming)
Screw or anchor into fiberglass
Thin fiberglass won't hold a screw well, so through-bolt with a backing plate when you can. In cored panels, drill the hole oversize, fill it with thickened epoxy and redrill once it cures. The epoxy keeps water out of the core and spreads the load.
What to use: Through-bolts + thickened epoxy
Guides
- How to countersink a screw (and how deep) (coming)
- Types of screws: the 14 you'll actually meet (and what each is for) (coming)
Sand or finish fiberglass
Sand gelcoat and fiberglass wet when you can, which keeps the dust down. Use 80-grit to prep for a repair, then finer grits to blend. Sand only with good airflow and a dust mask, and wash your skin and clothes after.
What to use: Wet-or-dry paper + N95 mask
Guides
- Sand fiberglass and gelcoat (safely) (coming)
- Sandpaper grit chart: which grit for which job (coming)
Safety basics
- Wear a mask, glasses and long sleeves. Fiberglass dust irritates skin, eyes and lungs. Use at least an N95 for cutting and sanding.
- Ventilate for resin. Polyester resin and gelcoat give off styrene, which can cause headaches and dizziness. Work outside or with strong airflow.
- Cut the power first. Before you cut a hot tub or a wired panel, have the power disconnected at the breaker.
- Clean up with a vacuum. Sweeping kicks fibers back into the air. Wash work clothes on their own.
Sources
- Drilling gelcoat: chamfer the hole edge with a countersink so it won't crack (Practical Boat Owner)
- DS0930DGX 9 in. diamond grit reciprocating blade for cast iron, fiber cement, masonry and fiberglass (Diablo)
- Surface preparation: polyester laminates, wax remover then 80-grit to a dull finish (WEST SYSTEM)
- Polyester gelcoat over epoxy: epoxy bonds to cured polyester laminate (Epoxyworks, WEST SYSTEM)
- Proper fastener bonding: oversize holes about 2x shaft for small fasteners, 1/4 in larger above 3/8 in, thickened epoxy (Epoxyworks, WEST SYSTEM)
- Common shop dusts: glass fiber and sanding dust, particulate mask, ventilation, wet sanding (WEST SYSTEM)
- Synthetic vitreous fibers ToxFAQs: skin, eye and lung irritation (ATSDR, CDC)
- Styrene: workers who make boats, tubs and showers are exposed; headache, fatigue, dizziness (OSHA)
- How are hot tubs made? Acrylic shell, fiberglass backing, foam and frame (Sundance Spas)