Fixing a plastic hull: what epoxy can and cannot do
Epoxy is great on fibreglass and wood, and a bad idea on most plastic boats. Learn how to tell what your hull is made of, what to strip, what to repair and what to leave alone.
Start with the material
The right repair depends on what the hull is made of. Plastic canoes and kayaks are usually one thick layer of polyethylene. Fibreglass boats have a glossy outer layer over layers of glass cloth and resin. Wooden boats are often plywood sealed with epoxy. Find out before you buy a single tin of anything.
Why epoxy fails on plastic
Nothing sticks well to polyethylene, and it also flexes. Epoxy is hard and stiff, so it forms a weak bond and then cracks and lifts off as the hull bends. That is why an old epoxy and paint job on a plastic boat often ends up lumpy, heavy and peeling.
Look before you fix
Do not repair what you have not seen. Clean the hull, strip any old coating, then look for real damage: cracks, deep gouges and thin spots. Often the plastic underneath is fine. A boat for learning does not need to be beautiful. It needs to be sound, light and easy to use.
Safety
Old paint and epoxy make dust and fumes when you sand or heat them. Work outside or with plenty of air, and wear a proper dust mask or respirator and eye protection. A heat gun and a flame burn skin and can warp the hull, so keep them moving and try on a scrap first. This is a first look, so read the product instructions too.
Cleaning up a coated plastic hull
Tap the jobs in the order you would do them. Each right tap locks in; a wrong tap gives you a hint.
Three kinds of hull wall
A coated plastic hull
The tools for the job
Study sheet
All the words in one place, by level.
Level 1: What is the hull made of?
Polyethylene wall
Most plastic canoes and kayaks are made from one thick layer of polyethylene, a waxy, tough plastic. It is waterproof all the way through and takes knocks well. It flexes, and paint and epoxy do not stick to it easily. A scratch on it leaves a pale line, and it feels slightly waxy.
Gelcoat
The thin, glossy outer layer of a fibreglass boat. It gives the colour and shine and protects the glass layers. It can crack and chip, and then water can get into the layers under it.
Glass cloth and resin
The strong part of a fibreglass boat: layers of woven glass cloth soaked in resin. Epoxy bonds very well to it, so cracks and holes here are repaired with new cloth and epoxy.
Plywood
Thin sheets of wood glued in layers. Wood soaks up water and rots, so a plywood boat is sealed with epoxy and then painted. Epoxy bonds very well to wood.
Epoxy coating
A thin, hard, waterproof layer that seals wood. It is the right job on wood and fibreglass, and the wrong job on a plastic hull, which needs no sealing.
Level 2: Read the old repair
Plastic shell
The real hull, made of polyethylene. If it has no cracks and no thin spots, the boat is fine, however ugly the outside looks. Your goal is to get back to it and look at it.
Old epoxy layer
A thick layer someone added over the plastic, maybe to change the colour or to hide scratches. On plastic it bonds badly, cracks as the hull flexes, and adds weight. It is the first thing to remove.
Old paint
The coloured top layer. It was put on over the epoxy, so it comes off with it. Paint on plastic often flakes, and the dust is harmful, so wear a mask when you sand it.
Lifted edge
A place where the coating has come away from the plastic. It shows that the bond is poor. If one edge lifts, the rest is probably weak too, so strip the whole coating, not just the loose parts.
Gouge
A deep scratch or cut in the plastic. Shallow ones are only cosmetic. A deep one, a crack or a thin spot is real damage. Repair it by plastic welding, or with a flexible epoxy made for plastics, not hard epoxy or body filler.
Level 3: The toolkit
Heat gun
Blows hot air. Used on low to medium heat it softens old paint and epoxy so you can scrape it off. Too much heat, or heat held in one place, warps the plastic. Keep it moving, and use it in the open air.
Scraper
A stiff blade or putty knife. Pull it across the warm coating to peel it off. Hold it flat so the corners do not gouge the plastic underneath.
Orbital sander
A sanding disc that moves in small circles. Coarse paper (about 80 grit) removes stubborn bits and fine paper (to about 220 grit) smooths the surface. Work up in steps and wear a mask.
Plastic welding rod
A rod of the same plastic as your hull, usually polyethylene. You melt it into a crack or gouge with a heat gun or welding tool, so the repair is plastic again and flexes with the hull. It works on most polyethylene hulls, so test on a hidden spot first.
Flexible epoxy
Epoxy made to bend a little, such as West System G/flex. Used for plastic repairs and filling gouges when welding is not an option. On polyethylene the surface must be prepared first, for example by a quick flame pass, and the makers say to test a small spot first.
Propane torch
Used for a very quick pass of flame over polyethylene. It changes the surface so that glue, epoxy or primer can grip. Keep the flame moving, because the plastic melts and warps fast. Practise on a scrap piece first.
Respirator
A mask with filters that protects your lungs from the dust and fumes of old paint, epoxy and sanding. A thin paper mask is not enough for this job, and you need eye protection as well.