Epoxy Barrier Coat Below the Waterline: Coats, Windows, Bottom Paint

An epoxy barrier coat below the waterline is a film thickness job, not a coat count job: you are building about 10 mil (0.25 mm) of dry epoxy over the whole wetted surface, which with a rolled two-part epoxy works out to 4 to 6 coats. Each coat goes on inside the previous coat's chemical-bond window or over a sanded surface, and the bottom paint goes on inside the barrier coat's overcoat window from the TDS, never onto fully cured glossy epoxy that has not been sanded. Here is how we run it on a stripped hull.

Prep before the first coat

Haul, pressure wash, strip or sand off the old antifouling, then let the laminate dry. Check it with a moisture meter against the topsides above the boot stripe; when the bottom reads close to the topsides you are dry enough to coat. Coating over wet laminate seals in the water that causes the blisters you are trying to prevent.

Sand the gelcoat with 80 grit until there is no gloss anywhere, then solvent wipe. Open any blisters with a rotary burr, let them drain and dry, and fill the craters. For scattered craters and gouges we use the Boat Suppliers Thick Epoxy Resin Kit, the 1:1 no-sag formula, because it stays in an overhead hole and sands flush the next day. Keep the hull above 50°F (10°C) for the whole job unless the primer's TDS says otherwise.

How many coats: the mil math

A rolled coat of two-part epoxy lays down about 2 mil dry when you roll it thin and tip it, so the count follows from the film build.

Sea Hawk Tuff Stuff is a high-build primer and gets there in fewer passes; its spread rate and mils per coat are on the TDS, so check them against the table rather than assuming 2 mil. For a 100% solids epoxy like West System 105, size the batch with the resin calculator against wetted surface area. A quick estimate for a planing powerboat is waterline length times beam times 0.85, so a 22 ft waterline with an 8 ft beam is roughly 150 sq ft (14 sq m) per coat.

Recoat windows: chemical bond or sand

Every two-part epoxy has a window after each coat where the next coat still bonds chemically to it; past it, the surface is fully cured plastic and the new coat only gets a mechanical bond, which means sanding.

The window length is on the hardener's TDS and it moves with temperature. For West System 105 with 205 or 206, the yard rule is that you can recoat without sanding as long as the previous coat has not fully hardened; once it has (Gougeon's figure is about 72 hours at 70°F / 21°C) you wash the blush off and sand. Hardener choice sets your pace: West System 206 Slow Hardener gives 20 to 25 minutes of pot life in a 100 g cup at 72°F (22°C), which is what you want rolling 150 sq ft in a warm shop, while West System 205 Fast Hardener runs 9 to 12 minutes and keeps curing down to 40°F (4°C). Both are 5:1 by volume with 105; use the pumps or a scale.

We lost a day on a 25 ft Contender last spring because the third coat went on Friday and nobody got back to it until Tuesday. The whole bottom got an 80 grit scuff and a wash before coat four. A nuisance, not a failure. Skipping the sand is the failure.

Which epoxy barrier coat to use

For a fiberglass hull, the fastest way through the job is a purpose-built epoxy primer. Sea Hawk Tuff Stuff Gray Epoxy Primer is a high-build two-part epoxy that uses microsheet silicate to build a waterproof film, and it dries fast enough that the whole system, barrier coats plus bottom paint, fits in two working days. The 1/2 gallon kit is spot-work scale; a full bottom is 2 gallon kit territory, with the TDS spread rate setting the kit count. Sea Hawk Tuff Stuff White Epoxy Primer is the same primer in white, the pick under a light bottom paint or as a contrasting coat. Alternating gray and white is an old yard trick: holidays show up immediately.

If you are already doing epoxy repair work on the hull, West 105 with 205 or 206 does the job as a straight coating. It is 100% solids so what you roll on is what you keep, but it needs the full 5 to 6 coats and it blushes, so plan the washes into the schedule.

Aluminum is a different job. Pettit AlumaProtect Epoxy Barrier Coat Primer is a two-part strontium chromate epoxy made for roughed bare aluminum, and it is the right primer there because copper antifouling on bare aluminum sets up galvanic corrosion. Do not put it on a fiberglass hull, and do not put a fiberglass barrier coat on aluminum.

Bottom paint over the barrier coat

The barrier coat TDS gives an overcoat window, the span after the final coat where antifouling bonds to it without sanding. Inside it, some primers want the epoxy still slightly tacky (the thumbprint test: you can leave a print but nothing comes off on your thumb), others want it dry to the touch. Miss the window and the epoxy cures to a hard gloss that bottom paint peels off in sheets at the first pressure wash, so past the window you wash, sand dull with 80 grit, solvent wipe, then paint. Ablative or hard antifouling makes no difference to any of this.

Most of the hull sanding is 80 grit on a DA. For hand scuffing between coats and the finer work at the boot stripe, the Wet & Dry Sandpaper Packs cover 120 grit and up on full sheets. Wear a fitted respirator with P100 and organic vapor cartridges for the sanding and the solvent-borne primer, nitrile gloves, and eye protection.

Frequently asked questions

What You Need For This Job

• Sea Hawk Tuff Stuff Gray Epoxy Primer: high-build two-part epoxy barrier primer for fiberglass hulls
• Sea Hawk Tuff Stuff White Epoxy Primer: same primer in white for the alternating-coat method
• Pettit AlumaProtect Epoxy Barrier Coat Primer: strontium chromate epoxy primer for bare aluminum hulls only
• West System 206 Slow Hardener: 5:1 hardener for rolling 105 over a big bottom in a warm shop
• West System 205 Fast Hardener: 5:1 hardener for cool-weather coating down to 40°F
• Boat Suppliers Thick Epoxy Resin Kit: 1:1 no-sag epoxy for filling blister craters before the first coat