Hi All: IF I remember correctly, old gel coat from the 70's and 80's, would blister, eventually if the cured gel coat film thickness got to be less than 20 mils. It can be thin to start with (i.e. not enough gel coat sprayed into the mold) or it can get thin by repeated sanding. Any way you get it, thin gel coat is going to blister. I am just doing my M-17 bottom and find the blisters in patches, (from vigorous sanding by PO's I'm guessing). Any time I take the bottom paint off down to the gel coat, I apply at least two coats of a barrier coat as a matter of course because if it isn't blistered, it will be the next time I haul out. Low quality gel coat, and/or improper application are also sources. Just my $.02 worth! Leif Eriksson New owner of very old M-17. ************************************** See what's new at http://www.aol.com