Hull Blisters (Osmosis): How to Tell Cosmetic Pimples From Serious Pox

Walk around the whole hull with a light held low and at an angle so bumps cast shadows. Look along the waterline, around the keel and near through-hulls. Run your hand over the surface; small blisters are often easier to feel than to see.

Mark each blister with tape or a marker and take photos with something for scale. Count them and note where they are. A few scattered or in one patch is very different from hundreds everywhere. Save the photos in BoatMend so you can compare next haul-out.

If the boat has been out for a while, lightly scuff the bottom with a coarse sanding block (with dust control and PPE). Raised spots show up as shiny or untouched patches you can mark.

With safety glasses on, pop a typical blister with the corner of a sharp chisel, held at arm's length. Note what you find:

• Dry, with paint underneath: a paint blister. Sand, prime if needed, and repaint.
• Liquid, often with a sharp vinegar smell, gelcoat dome over sound laminate: a typical osmotic gelcoat blister.
• Wet, white or stringy fibers under the gelcoat: the blister reached the laminate. Plan for a deeper repair or a pro.

Tap around each one with a plastic mallet or a screwdriver handle. Solid laminate gives a sharp click. A dull thud means a void or delamination, and the damaged area is bigger than the dome. Mark the edge of the dull area.

Surveyors use meters to compare wetness from place to place, for example the bottom against the topsides. But most handheld meters were designed for wood or building materials, copper bottom paint and other additives can throw readings off, and older hulls often read "wet" without ever blistering. A high reading alone is not a diagnosis, and a meter can't predict blistering on a hull that has none. Treat readings as one clue alongside what you see and hear.

• A few blisters, in one area or scattered, laminate solid around them: repair them one by one (next steps). Repairing each blister also cures it.
• Many blisters across the bottom, blisters into the laminate, or large dull-sounding areas: stop and get an independent survey before spending money (see "When to call a pro").

Open every marked blister and wash away the liquid. With a small sanding disc, grind away the broken dome and bevel the edges into a shallow saucer. A common rule of thumb is a dish about 20 times wider than it is deep. Keep widening (not deepening) until the gelcoat edge is firmly bonded all the way around. Healthy laminate looks dark and translucent when wet; damaged laminate looks white and fibrous.

Scrub every dish clean with fresh water and a stiff brush. Then let the laminate dry as long as you can. If the boat is coming out for the winter, the easiest plan is to open and scrub the blisters at haul-out and fill them in spring. If you can't wait, blot each dish repeatedly with isopropyl alcohol on paper towels until the towel comes away clean.

Tape a square cut from a clear freezer bag over a few blisters, seal all edges with duct tape, and wait about three days in warm weather. Fog or droplets on the inside of the plastic mean the laminate is still wet: give it more dry time. Dry plastic means you can fill.

Wipe each dish with alcohol just before filling. Brush a thin coat of plain mixed epoxy into the dish, let it get tacky, then fill with epoxy thickened with colloidal silica to a peanut-butter consistency. Squeegee it smooth and slightly proud. Do not use polyester resin, body filler, or hollow fillers like microballoons or talc below the waterline. Deep blisters that went into the laminate get layers of fiberglass cloth wetted with epoxy; if that sounds like more than you want to take on, have a pro do it.

Coat each repair and a little beyond with a couple of coats of unthickened epoxy or the barrier coat your paint maker recommends. After it cures, wash off the waxy film (amine blush) with water and an abrasive pad, sand it fair, and apply bottom paint (see our bottom paint guide). Don't put gelcoat over epoxy repairs.

Compare against your photos. A few new blisters can be treated the same way. A fast-growing number is a reason to get a survey.