Had this happen to my boat, just lots of small blisters that developed over time in small sections of the topsides. The culprit was probably someone at the yard when the boat was being built got air bubbles in the auto catalyst/resin mixing 'gun' when applying. When I excavated the 'blisters' I found a lot of still uncured resin in the 'cosmetic' (non-structural) matting layer under the gelcoat layer.
For the repair, Id suggest that you fully grind out all the area until youre sure that youre into 'solid' glass. Then, epoxy-layup small layers of cloth (12 oz. cloth goes in and builds thickness quite fast) leaving about 1/8" space which will be then filled with epoxy filled with "CaboSill" or WEST System "colloidal silica" filler --- to 'mayonaise or peanut butter consistency', then flat sand with a 'long board' to a perfect surface to match the surrounding surface. Then you can eyeball 'match' the gelcoat - apply with a Preval Spray gun, etc.
One of the 'best' videos on how to do this type of repair that Ive ever found is: "Professional Gelcoat Repairs without a Mess" by John Gabriel
www.boatrepairvideos.com This video is 'really' worth the cost.
Here's the Preval aerosol powered spray gun:
http://www.jamestowndistributors.co...t.do?pid=2155&familyName=Preval+Spray+Gun+Kit
This type of repair isnt all that big of a deal. If you DIY repair and are 'careful and diligent' youll probably save at least $1000.
If you really want a perfect gelcoat color match, cut a plug out of the hull and send it to a gelcoat supply that has a color matching computer ... get 'paste' and/plus already mixed gel ready for 'spraying'. Usually ~$100 for the color matching and another $100 or so for the gel ... check their 'minimum charges' first.
I prefer to order my color matched gel from Gelcoat Products in Seattle, WA.
If youre over 40 years of age, dont depend on YOUR eyes to do the color matching if you do the color matching yourself; rather, get a YOUNG person with still good color perception to be the final judge on the color match ... once you are over ~40 you dont see 'reds' very well.
If DIY is not your forte, seek out a local gelcoat 'artist' to do this for you.