Simon Zorovich of Matawan, New Jersey asks:
The after effects of Hurricane Sandy are still being felt here in the Northeast in that competent fiberglass repairmen are all booked up with jobs through the upcoming boating season. Many boatowners have therefore been forced to contemplate repairing their storm-damaged boats themselves. I have read some informative articles by Don Casey on the subject, and my question is this: should I use a polyester resin or epoxy to repair an enormous gouge in my boat? As you can see in the photo (below), it's about 2ft by 6in and cuts well into the fiberglass cloth, but not all the way through the laminate.
Don Casey replies:
Two decades ago I found myself in a similar situation in the aftermath of Hurricane Andrew and, like you, ended up doing the repairs myself. I’m happy to say they remain strong and undetectable to this day. The advantage of using epoxy for repair work is that the bond to the existing laminate will be about 20 percent stronger than when using polyester resin, which provides a welcome margin of error for the amateur worker. Professionals tend to view the extra adhesion as superfluous and not worth the high price of epoxy, which costs two or three times as much as polyester. Epoxy is also incompatible with gelcoat, so if you plan to finish the repair with gelcoat rather than paint, you should use polyester or perhaps vinlyester resin instead.
As for the layup process, in both cases you need to grind away all of the damaged laminate and create a 12:1 bevel around the perimeter. If you use polyester, you can lay down alternating layers of 1.5-ounce mat and 6-ounce cloth, unless you have reason to follow a different schedule. Epoxy repairs are usually made with woven fabrics only, as epoxy is incompatible with the binder in common fiberglass mat. Special epoxy-compatible mat can be obtained if you decide you need it. Your new laminate layers should go down in larger-to-smaller order, because laying down the largest piece first maximizes the area of the secondary bond between the old and new glass. The bond between subsequent layers will be chemical in nature and thus considerably stronger.
Because the perimeter bevel greatly enlarges the size of the repair, cutting out the damage and fashioning the repair from inside can be an attractive alternative, if you have interior access to the damaged area. Whatever method you use, if you grind well, mix the resin correctly, and compress all the air out of the laminate, your repair will be indistinguishable from the work done by a pro, cosmetic appearance excepted. If you fail cosmetically, you can always get professional help next season.