Stripping Old Bottom Paint: Chemical Strippers, Blasting and Sanding

Have the yard pressure wash the bottom right after haul-out on their wash pad, where runoff is captured. Loose paint often comes off in that wash. Let the hull dry fully before chemical stripping or sanding.

Fiberglass, aluminum and wood each need different products. On aluminum, use only a stripper labeled for aluminum and never sand or blast through the factory coating without a plan to re-prime with the paint maker's aluminum system (see our aluminum hull corrosion guide). Ask the previous owner or look in the records for what paint is on the bottom and whether the boat ever had a barrier coat.

Practical Sailor's stripper tests found big differences between products, paint types and dwell times, and found that the longest wait was not always the best. Try one or two strippers on a patch about a foot square. Note how many layers come off and at what time. Some strippers need warm weather to work, so check the temperature range on the label. Practical Sailor found results vary from boat to boat, so if the first product disappoints, try another before giving up on chemical stripping.

Lay plastic sheeting under and around the work area and weigh it down. A blue tarp that you have walked on is no longer waterproof, so use material you can bundle up and throw away. Keep rinse water from cockpit drains and through-hulls off the work area (short pieces of hose in the through-hulls help). Put on all your PPE before you open the first can. Read the stripper's safety data sheet.

Work in sections you can scrape within the product's dwell time, often starting low and working along the hull. Lay it on thick, as the label says. Some products come with a cover paper or film that keeps them from drying out; use it. Never let a fast stripper sit longer than the label allows on gelcoat.

Use plastic scrapers or metal scrapers with all corners rounded off, held at a low angle, so you lift the softened paint without gouging the gelcoat. Scrape the sludge straight into a container or onto the sheeting, not onto the ground. Expect to repeat the stripper on stubborn areas and on the keel and waterline.

Wash off stripper residue with the cleaner or rinse method on the label. Some strippers need a specific wash before primer will stick. Let the hull dry.

Ask the contractor how many fiberglass boats they have done, what media they use, how deep they plan to go and how they contain and dispose of the waste. Practical Sailor recommends asking for references. A careful operator stops at the last thin layer of paint and leaves it for machine sanding rather than blasting into the gelcoat. Make sure the yard allows it and that your neighbors' boats are covered.

Use a random orbit sander hooked to a vacuum, with coarse discs for leftover paint and finer ones for final prep. Stop as soon as you reach gelcoat (or bare laminate, if you are fixing blisters). Never use an angle grinder or belt sander: they gouge the hull in seconds. Sweep or vacuum dust and chips; do not hose them away.

This is your best look at the hull in years. Check for blisters, cracks, voids, damage around the keel joint, through-hulls and transducers, and soft spots (see our hull blisters guide). On a sailboat, look at the keel-to-hull seam. Fix problems now.

A fiberglass hull that has been in the water for years can hold moisture. Epoxy barrier coats need a dry laminate. Many owners have a surveyor take moisture readings or follow the paint maker's moisture guidance before coating.

Don't leave a bare hull sitting unprotected for long; get at least the first coat of primer on as soon as the surface is ready and dry. On fiberglass, the paint maker's epoxy barrier coat system is the usual choice after a full strip. Follow its overcoat window exactly: antifouling must go on within the time the primer label gives, or you have to sand and re-prime.

Choose a paint that is compatible with the primer, your water and your hull material, and apply it as described in our bottom paint guide. Record the products and dates in BoatMend.

Bag sludge, chips, dust, sheeting, scrapers, rags and used PPE and take them to the yard's hazardous waste collection or your local household hazardous waste site. Paint cans with residue are hazardous waste unless emptied as the label says. Never let anything wash into storm drains or the water.