Short answer
Epoxy bonds better when you repair or bond onto an old, cured hull. Polyester is cheaper and works well for building thick layers or applying gelcoat over polyester. Most GRP boats are built in polyester. Rule of thumb: epoxy can go over polyester, but polyester shouldn't go over epoxy.
Comparison
| Polyester | Epoxy | |
|---|---|---|
| Used for | Building new boats, thick laminates, gelcoat | Repairs, bonding, moisture sealing below the waterline |
| Adhesion to old laminate | Poorer | Better |
| Mixing | Hardener (MEKP) 1 to 4 % by weight | Fixed ratio, e.g. 5:1 by volume (West System 105) |
| Glass fibre | Chopped strand mat and woven cloth | Woven cloth and biaxial cloth |
| Smell and fumes | Styrene, strong smell | Faint smell, but a strong allergen |
| Price | Lower | Higher |
| Top layer | Gelcoat or topcoat | Paint or primer, gelcoat only as directed by the manufacturer |
Mix correctly
Polyester: the MEKP hardener is dosed as a percentage of the resin weight, 1 to 4 %. Less hardener gives slower curing and more gives faster. If you go outside the range, the laminate will be weaker. The can has a table showing how much hardener to use at different temperatures.
Epoxy: the ratio is fixed. Don't change it to speed things up. West System 105 is mixed 5:1 by volume. Hardener 205 needs at least 4 °C and 206 at least 16 °C. If you want faster curing, switch 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 form a waxy film 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 and paint. Gelcoat over epoxy also works, but then you follow the epoxy manufacturer's instructions exactly.
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 works if the epoxy is properly prepared, following the epoxy manufacturer's instructions. Painting is easiest.
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