How to Paint a Boat Bilge (Prep, Paint Choice and What to Leave Bare)

Pump out the water, remove loose gear, and soak up any oil with absorbent pads before you wash. Never pump oily water overboard; it is illegal and the bilge cleaning guide explains how to dispose of it. Then scrub with a bilge cleaner and fresh water, rinse, and vacuum out the rinse water. Repeat until the water comes out clean. Let the bilge dry completely, with the hatches open and air moving.

Wrap or tape off everything that must not get paint:

• Float switches, bilge pumps and pump strainers. Paint can glue a float switch in the "off" position.
• Wiring, terminals, fuses and bonding wire connections.
• Hoses, hose clamps, seacock handles and other moving parts. A seacock that is painted shut is a safety problem.
• Fuel tank labels, engine and drive serial number plates, and other required labels. You or a surveyor will need to read them later.
• Any anodes, fuel and water tank fittings, sensors and transducers.
• Limber holes (the drain holes through the frames). Keep them open so water can still reach the pump.

If you can easily remove the pump and float switch, take them out and label the wires instead.

With the bilge clean, look closely at what you have. Note any cracks, soft spots, wet or dark wood, and stains that point to a leak. Then check the old coating: Interlux says that if the existing paint is cracking, peeling or lifting, all of it has to come off and the surface must be primed. If the old paint is firmly attached, you only need to clean and sand it. Old paint that is flaking like the bilge in the cover photo has to come off completely; painting over it just peels with it.

Even a bilge that looks clean usually carries a film of oil and wax. Wipe every surface you will paint with the solvent or surface prep product the paint maker names, using clean rags and turning them often so you lift the contamination instead of smearing it. TotalBoat notes that most adhesion failures come from grease that was not fully removed or a surface that was not fully dry.

• Old paint in good condition and bare fiberglass: both makers call for sanding with roughly 220 to 320 grit after the solvent wipe.
• Flaking or peeling paint: scrape and sand it all off, then follow the bare-surface prep.
• Bare wood: sand smooth and wipe clean; check the paint label for a first coat thinned more than usual.

Vacuum, then wipe with solvent or a tack cloth until a clean rag stays clean. Sanding dust is just as bad for adhesion as oil.

Read the label and data sheet for your exact product; they differ.

• Alkyd bilge enamel (for example Pettit EZ Bilge or Interlux Bilgekote): a one-part, high-solids paint made for bilges. Pettit says EZ Bilge resists fuel, engine oil, grease and antifreeze. Simple to apply and recoat.
• Epoxy-based bilge paint (for example TotalBoat TotalBilge): dries harder and glossier. TotalBoat says it resists moisture, oils and solvents, can go on bare fiberglass, gelcoat, wood and epoxy, but is not recommended on aluminum or steel.
• Epoxy barrier coat under the bilge paint: Interlux suggests InterProtect or an epoxy primer under Bilgekote for extra protection against moisture soaking in, and TotalBoat offers a barrier coat primer as an option. It means more steps but is a good idea where water sits all the time.

Color: White or light gray makes leaks and dropped parts easiest to see.

• Bare metal: TotalBoat says a primer is mandatory on metal, and its epoxy bilge paint is not meant for aluminum or steel at all. Use the system the maker lists for your metal.
• Bare wood: prime as the label directs.
• Bare fiberglass: some systems prime it, some do not. Interlux notes that the rough back side of molded fiberglass (what you see in most bilges) does not need primer before Bilgekote.

Watch the primer's overcoat window. Sand primer lightly if the label says so, then remove the dust again.

Interlux recommends two fans: one blowing fresh air in and one pulling stale air out. Put them outside the bilge or at the hatch openings, not down in the fumes, and keep extension cord plugs out of the space too. Good airflow protects you and also helps the paint dry properly. Put on your respirator, gloves, eye protection and coveralls now, before you start brushing.

Stir the paint thoroughly; TotalBoat warns that the heavy solids settle at the bottom of epoxy bilge paint. Cut in corners, frames and around fittings with a small brush, then roll the flat areas. Keep coats thin: TotalBoat says a heavy coat traps solvent underneath and will not dry, and gives about 3 mils wet per coat for its product. Most makers call for one to two coats on fiberglass and more on some surfaces; follow the label. Thin only with the thinner the label names, and only as much as it allows.

Let each coat dry for the full overcoat time on the label before the next. The times change a lot with temperature: TotalBoat lists hours between coats that roughly double from hot to cool weather. Paint early enough that the coat can dry before evening dew. Sand lightly between coats if the label says so, and remove the dust each time.

Pull the tape while the last coat is still slightly soft, or score the edge with a razor so the paint does not peel up with it. Let the final coat cure for the time on the label before bilge water, fuel, oil or traffic touch it. Then reinstall the pump and float switch, reconnect the wiring, and test that the float switch moves freely and turns the pump on.

Wipe the bilge often and keep an absorbent pad under the engine. Look down there every time you use the boat: a new drip or stain on a clean, light surface is the earliest warning you will get. Log the date, paint used and color in BoatMend so the next touch-up matches.

A BoatUS writer describes a decades-old engine compartment that still looks new because the area under the engine gets towel-dried after every trip: lower humidity means less rust on engine parts, and a dry, painted surface makes a new leak obvious. BoatUS also suggests washing the bilge with fresh water and clearing debris about once a month, because loose bits clog float switches and pump strainers. Test the pump from the helm switch and by lifting the float each time.