BoatsExplained.com - Saving Gel Coat from Blisters

Blisters can be broadly divided into shallow and deep. Shallow blisters, meaning those confined to the gel coat, can simply be broken open (a countersink drill can be useful for this) and, once they're dry, filled with an epoxy filler. The chances are that more will appear later. This doesn't mean that your treatment was unsuccessful - for these will be a fresh batch that would have popped up anyway.

What you're now involved in is an ongoing maintenance programme whereby you hope to contain the problem for a matter of years - with luck, indefinitely. Avoid sanding the bottom of your boat. Large blisters will be opened up but there will be many more smaller ones which will just be shaved thin and remain as perfect sites for further development.
Deep blisters are more difficult. Many people attack them with a grinder but that's unnecessarily destructive, inevitably destroying parI of the structural moulding. Also grinding seldom opens the cavity fully, because the dusty rough surface masks the edge, so a halo is left to cause trouble later. A better and less destructive way is to open the blisters with a chisel, a small grinding bit or a router set shallow. Again, use your countersink bit on the smallest ones.
Always repair large voids with glassfibre.

There's another even kinder method I've used on my own boat for ten years. Deep blisters are basically delamination between layers and still retain considerable strength.

So why not bond them together? The technique here is to pierce and drain the bulges. Later, drill shallow holes around the edge and in the middle, and inject thin epoxy. Some people recommend a paste but to my mind it should be runny enough to penetrate the capillary paths surrounding the bulge and also the extremities of the void.

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