Short answer
Most GRP boats are built with polyester. If you are repairing or bonding to an old, cured hull, epoxy adheres better. Polyester is cheaper and works well for building up thick layers or applying gelcoat over polyester. The simple rule: epoxy can go over polyester, but polyester should not go over epoxy.
Comparison
| Polyester | Epoxy | |
|---|---|---|
| Used for | Building new boats, thick laminates, gelcoat | Repairs, bonding, moisture barrier below the waterline |
| Bonds to old laminate | Worse | Better |
| Mixing | Hardener (MEKP) 1–4% by weight | Fixed ratio, e.g. 5:1 by volume (West System 105) |
| Glass fibre | Chopped strand mat and woven roving | Woven cloth and biaxial cloth |
| Smell and fumes | Styrene, strong smell | Faint smell, but a strong allergen |
| Price | Lower | Higher |
| Top coat | Gelcoat or topcoat | Paint or primer, gelcoat only per the manufacturer's instructions |
Mix correctly
Polyester: the MEKP hardener is dosed as a percentage of the resin's weight. Resin Library specifies 1–4 %. Less hardener gives slower curing, more gives faster. Outside this range the laminate is weaker. Always follow the temperature chart on the tin.
Epoxy: the ratio is fixed and should not be changed to speed things up. West System 105 is mixed 5:1 by volume. Hardener 205 requires at least 4 °C, 206 at least 16 °C. If you want faster curing, change hardener or heat the workspace.
Choose by the job
| Job | Choose |
|---|---|
| Hole or crack in the hull, see repairing a hole in a fibreglass boat | Epoxy |
| Bonding fittings or filling old screw holes | Epoxy |
| Dried hull after osmosis | Epoxy |
| Aluminium, see repairing an aluminium boat | Epoxy, preferably a toughened type |
| Small gelcoat damage, see repairing gelcoat | Gelcoat (polyester) |
| Large new laminates in a polyester boat, e.g. a new cabin sole | Polyester or epoxy |
Finish coat over epoxy
Cured epoxy can develop a waxy film (amine blush) that stops the next coat from bonding. West System says to remove it with water and an abrasive pad. Then sand with 80-grit paper. After that you can paint. Gelcoat over epoxy is possible, but follow the epoxy maker's instructions step by step.
Safety
- Styrene from polyester is highly flammable and irritates the eyes and airways. Work with good ventilation, preferably outdoors, and use a respirator with an organic vapour filter.
- Epoxy is a sensitiser. The Swedish Work Environment Authority states that allergy can develop after a single splash. Use nitrile gloves and safety glasses.
- Sanding dust from both materials must not be inhaled. Use a respirator with a particle filter.
- Don't mix more than you can use in time. Mixed resin in a cup gets hot and can start to smoke.
Frequently asked questions
Can I apply gelcoat over an epoxy repair?
It's possible, but the gelcoat only bonds if the epoxy is properly prepared. Follow the epoxy manufacturer's instructions. The easiest option is to paint.
Why won't my polyester cure?
Usually it's too cold or too little hardener. The surface can also stay tacky if the resin has no wax and is exposed to air.
Can I mix epoxy by eye?
No. The wrong ratio gives a repair that never fully hardens. Use dispensing pumps or scales.
Sources
- User Manual & Product Guide (105/205/206, ytförberedelse, säkerhet), West System westsystem.com
- Polyester resin manual (MEKP-dosering), Resin Library resinlibrary.com
- Fibreglass Boat Repair & Maintenance (manual 002-550), West System eu.westsystem.com
- Epoxi – allergirisker och skydd, Arbetsmiljöverket av.se