When you’re buying a used fiberglass boat, one of the biggest red flags you can come across is osmosis. It might sound like a minor cosmetic issue, but in reality, it can turn into one of the most expensive and frustrating problems an owner can face. Osmosis occurs when water seeps through the gelcoat and reacts with the resin inside the fiberglass hull. This chemical reaction forms pressure pockets that appear as blisters on the hull’s surface and once those blisters start, they rarely stop on their own.

While osmosis develops slowly, it’s a clear sign that water has penetrated where it shouldn’t. Over time, the trapped fluid can weaken the laminate layers, leading to soft spots, delamination, and eventually structural issues. Many sellers will brush it off as “just cosmetic,” but make no mistake treating a hull with osmosis properly can involve stripping the entire gelcoat, drying the hull for months, and rebuilding the protective layers with epoxy coatings. It’s an expensive, time-consuming process that can easily cost thousands and put your boat out of commission for am extended period of time.

Visually, osmosis can show up as small blisters or bubbles under the gelcoat, especially around the keel and underwater fittings. If you tap the hull and hear dull thuds rather than crisp, solid sounds, it could mean the laminate has already started to separate. Some owners try to hide the issue under fresh antifoul or paint, so don’t be fooled by a shiny hull alone! If a deal seems too good to be true, it probably is. Always insist on an inspection of the boat, and walk away from anything showing widespread blistering or evidence of past repairs unless the price truly reflects the work needed.

The reality is, there are plenty of clean, dry-hulled boats on the market that won’t saddle you with this hidden headache. Unless you’re deliberately taking on a restoration project and know exactly what you’re in for, a boat with osmosis is best avoided. It’s one of those problems that looks small at first but can quickly grow into a major financial sinkhole. In short... steer clear and keep shopping. Your future self (and your wallet) will thank you.

This information is provided as general guidance for boat buyers. Always seek the advice of someone experienced and qualified to discuss your options before purchasing or repairing a vessel.