What paint should I use on my boat's bottom, and how do I choose the right type?
My 28-foot fibreglass cruiser stays on a saltwater mooring from April to October, and I don't know what antifouling the previous owner used. Should I choose hard or self-polishing paint, and can I put it straight over the existing coating?
Answer
For most cruising boats kept afloat in saltwater, start with a compatible self-polishing antifouling; choose hard antifouling when speed, regular scrubbing or trailer contact calls for greater abrasion resistance. Before buying, identify the hull material and existing coating, then match the paint to local fouling, cruising speed and time afloat.
Check hull and existing paint
Check the previous owner's invoices, paint tins or yard records. Colour alone cannot identify a coating. Fresh antifouling over an incompatible or poorly bonded layer can leave the whole system peeling.
- Fibreglass: conventional marine antifouling is usually suitable over the correct primer or a sound, compatible existing coating.
- Aluminium: use only a coating system explicitly approved for aluminium. Ordinary copper-based antifouling can cause galvanic corrosion; check saildrive legs and outboard brackets separately.
- Steel: antifouling needs a sound anticorrosive primer system beneath it.
- Wood: choose a system approved for the timber and construction; traditional soft antifouling remains an option for suitable wooden hulls.
Look for flaking, thick cracked layers and exposed primer. Investigate blisters or corrosion before painting. Antifouling will not repair them.
Compare antifouling types
Hard antifouling leaves its binder behind as the active ingredients are released. Its resistance to mechanical wear suits many fast motorboats, regularly handled trailer boats and racing yachts whose owners want a carefully smoothed finish. Spent paint builds up, making preparation and eventual removal part of the job.
Traditional soft or eroding antifouling gradually loses binder along with its active ingredients. This reduces build-up, but the surface is less tolerant of rubbing, frequent scrubbing and trailer rollers.
Self-polishing antifouling renews its surface through a controlled polishing process. It suits many cruising yachts and displacement motorboats. Check each product's speed limits, permitted drying periods and suitability for boats that spend long spells stationary.
Shop terminology overlaps: "ablative", "eroding" and "self-polishing" are not precise substitutes for one another. Read the product's technical data sheet. Hard paint is not automatically less effective against fouling, and copper percentage alone will not tell you which tin will work best.
Match paint to boat use
Ask the yard and owners on neighbouring moorings what stayed clean through the previous season. A coating that works well in cool, moving water may struggle in a warm, sheltered marina with heavy slime or barnacle growth.