How to Repair Fiberglass Blisters

How-To - Code Flags

Recently, I was asked to write up a "how-to" tutorial on repair of fiberglass hull blisters. Several messages have recently appeared on the SailNet Chrysler and Mutineer eGroups discussion forums regarding this problem which all fiberglass boat owners have to deal with.

To write this tutorial, I've combined my own experience dealing with blisters with an excellent writeup provided by Roger Otterson.

Additionally, I've provided links to some other resources for hull blister repair information at the bottom of this tutorial.

Please keep in mind, as you read this tutorial, that there are MANY approaches to blister repair and products on the market designed specifically for blister repair. According to most knowledgeable people, one can never be 100% certain that blisters will not reemerge, once they have appeared, irregardless of how aggressively you treat the blisters.

Fiberglass blisters, or "boat pox", are caused because the gel coat is slightly porous, and absorbs water if left soaking for long periods. Blisters may also form if the boat fills up and sits with water in it, or under trailer supports where the hull does not have the opportunity to dry out.

These blisters are full of water under osmotic pressure and can burst forcefully if you "pop" them. They are primarily an aesthetic problem, although water submersion can eventually lead to delamination, which is a much more significant problem than "boat pox."

Here is the step-by-step procedure I use for repairing hull blisters:

  1. Open up the blisters with a knife. Make sure to stay well away from the blisters when you pop them, as the liquid inside the blisters is acidic -- you definitely don't want to get any of it in your eyes.
  2. Let the blisters dry out completely.
  3. Sand the blisters down to the fiberglass matting, until no sign of the blister is apparent.
  4. Clean the area with acetone or similar product.
  5. Fill the blistered area with an epoxy-based blister repair product. There are many epoxy products on the market. Perhaps the most well known product is West Systems. For this step, I use a product called MarineTex, which is available at most marine hardware stores.
  6. When the epoxy is completely dry, sand and fair down the surface so that it is nice and smooth.
  7. Once the repair has been faired out, clean the area with acetone or similar product.
  8. Apply a barrier coat with a roller.
  9. Once the barrier coat is dry, apply a bottom paint with a roller.

SAFETY TIP: It is wise to use gloves, long sleeves and pants, and protective eyewear throughout the blister repair process.

Here are some additional links to blister repair information. There is probably more information here than you want to know, but it doesn't hurt to hear from the experts!

Here are some links to blister repair product manufacturers. I'm certain that there are MANY other products out there, but these are the two that I'm familiar with.

Many thanks to Roger Otterson for his excellent writeup on hull blister repair, as posted to the Mutineer eGroups discussion forum. Roger sails his C-15 Mutineer in Bell, Florida. If you would like to ask him any questions about this process, send an e-mail message to

This page last updated on Friday, May 19, 2000.