Preparing Your Boat for Bottom Paint

If your boat's bottom paint has deteriorated as badly as this one, it will likely need to be sanded thoroughly with a drywall sander, then pressure-washed, before it can be repainted.

What should I do to prepare my boat for bottom paint? It is a question neophyte boat owners ask with some trepidation, but the answer could be as simple as pressure washing followed by a light sanding and solvent wipe. If that is all you have to do, just make sure to wear appropriate protective gear. Even old bottom paint has poisons that can be inhaled or absorbed through skin, so it is important to protect yourself from sanding dust and from solvents, and clean up immediately afterwards.

Most boat yards will pressure wash the hull for you as soon as the sling lift pulls it from the water, and that is always a good way to start — get the growth off before it dries on and becomes more difficult to remove. Another advantage of pressure washing the hull immediately upon haulout is that it allows you to examine the hull for cracks and blisters. Blisters in particular can often best be seen when they are wet, and may shrink and blend in to the dry hull. If time allows, check the hull after a day has passed to see if water is still coming out anywhere — this can be a good way to find cracks, keel separation, or through hull fittings with problems.

Check for Blisters

Hull blisters in fiberglass boats are generally a cosmetic, not structural, problem. Small ones can safely be left alone for later treatment at the next bottom job, but if there is significant blistering on a boat, it should be repaired before bottom painting. Water seeps through gelcoat and fiberglass resin over time, and a chemical reaction results that slowly erodes the plastics while producing internal pressure that stretches the hull skin, forming the blister. Gelcoat can also be blistered by excessive application of the heat gun during shrink wrapping. If gelcoat blistering is widespread, it may be worthwhile to have the gelcoat peeled by a professional with a marine shaver designed to cut off gelcoat. If peeling is not needed, individual blisters can be repaired by grinding and sanding out the area and then rebuilding the laminate with epoxy reinforced by glass fibers and fillers. Similarly, any defects or cracks around keel joints can be repaired and faired with thickened epoxy.

Your boat's bottom will be beautiful once again after a fresh coat of bottom paint.

Novice boaters may assume that a fiberglass hull encased in gelcoat is impermeable to water, but it is not. Water migrates though plastics, even modern vinyl ester gelcoats, via osmosis, and the result is a blistered hull. If the initial layup did not completely soak the fiberglass mat, water will fill any voids. Thus, the susceptibility of a given hull to blistering was determined on the day the gelcoat was sprayed into the mold and the glass laid on it. A hull that has blistered once will probably do it again, and a hull that has been in the water for a long time without blistering may never do so, or a change in environment may cause a sudden onset of the problem.

Epoxy barrier coat has been shown to resist water intrusion very effectively, but even barrier coat is not completely impervious to water. Actually, a layer of paraffin wax proved to be the most "waterproof" barrier the West Systems testers could find, in a test of moisture exclusion.

Protecting With Barrier Coats

Gelcoat cracks, blisters, and the desire to smooth and protect repairs can lead boat owners to want to cover their boats with an epoxy barrier coat such as Interlux Interprotect 2000E/2001E, West System epoxy with 422 barrier coat additive, Water Gard 300, Awlgrip's Hullgard Epoxy Primer, Pettit Protect, or MAS epoxies. There is some debate about the effectiveness of barrier coating, probably because people have had bad experiences in which a barrier coated boat blistered again.

If there are voids in the hull layup, that is almost inevitable for a boat stored in the water, and even a good barrier coating will serve only to delay the problem. Rapid barrier coat failures are generally the result of improper surface preparation, poor application conditions, or improper application techniques.

Barrier coat hull prep begins with removing undesired material. That can mean peeling the gelcoat with a machine or just removing old paint by soda blasting. Some masochists sand or grind this stuff off, but a professional with a soda blasting machine will really save a lot of time and effort. Once all foreign material is completely removed and the hull is fair and smooth, check it for trapped moisture with a moisture meter. You can find good tips on how to read (and avoid misreading) a moisture meter here.

If a hull is wet, it needs to be dried before barrier coating. Drying can be a slow process, but can be sped up with fans and heaters. If small areas are wet and can be effectively sealed off, vacuum bagging and applying a sustained vacuum to the affected area is probably the most effective fiberglass drying technique.

Once the hull is prepared for barrier coating, apply it according to manufacturer's instructions.

Common barrier coat application pitfalls to avoid

  • Not reading the instructions. Did I mention that you should read all of the instructions?
  • Not having backup rollers and materials on hand and readily accessible. No sense fumbling around for things while epoxy is curing if you drop your roller or something.
  • Applying barrier coat in windy conditions with blowing dust.
  • Getting the hot coating window wrong. Each successive coat will bond with the previous coat if applied before the previous coat has fully cured, while it is still tacky. Test the product you are using on something other than your boat, a dinghy perhaps, to see if it cures in the time you were expecting.
  • Misjudging the required number of layers to achieve the manufacturer's specified thickness.

If different colors of epoxy barrier coat are available for the brand you have chosen, it can be useful to alternate colors with each layer. When coating around boat stands or keel chocks, stop the first coat a short distance away, then each succeeding coat a bit further out, so you can see what you are doing as you fill in that area later.

It is generally recommended that the first coat of antifouling bottom paint be hot coated onto the last layer of barrier coat before it cures so that the two form a chemical bond. Be sure the barrier coat you are using is compatible with the paint you have chosen, and read the instructions to find out whether it is best to hot coat it or wait until the barrier coat is fully cured then sand it down for bottom painting.


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