Big piece or small?
"Many books on fiberglass repair, including one of yours, have drawings showing how to lay up cloth and mat over a tapered repair area, like a hole. The repair always begins with a small piece of cloth at the bottom, and as the layup continues, the pieces get larger. This makes sense to me, because a layup schedule doesn’t depend on just one interface bonding. Going from smaller to larger would seem to suggest that you are matching the shape of the tapered hole you are repairing. However, one epoxy manufacturer suggests a laminate schedule that begins with the largest piece and finishes with the smallest. Which approach is the right one for laying up material to cover a hole, stringers, and so forth?"
-- Jeff Field, Greenville, South Carolina
Don Casey replies: I’ll answer by quoting from the newest edition of my This Old Boat, which will be available in stores this summer: “You may encounter conflicting guidance—including in the previous edition of this book—on whether to apply the largest piece first or the smallest piece first. Here is the definitive answer to that question. If you are using either polyester or vinylester resin, put down the largest piece first. This will be mat, and it will give you the best bond to the underlying surface. If you put down the smallest piece (also mat) first, the next layer, which will be cloth, will not have mat between it and the existing laminate beyond the perimeter of the underlying piece. This will make for a weaker bond.
“If you are using epoxy resin, the order of the laminates does not actually make any difference in the strength of the repair for reinforcing or bonding. Small first tends to yield a neater appearance where the repair will not need to be faired. But where the larger layer drops over the edge of the one beneath, you may have a problem with creating a void and with the thinner epoxy resin draining through. This is avoided by laminating in a large-to-small order. The relatively recent development of epoxy-compatible mat also makes a large-to-small schedule using mat between the cloth layers a possibility.
“When filling a depression or doing reconstruction, the intuitive order is small to large, but the problem here is that we are trying to replace cut-away or ground-away fabric, and the new fabric, except for the bottom piece, will attach to the original material only at the perimeter—in effect, a butt joint. So the largest piece should go into the cavity first to maximize the surface area of the secondary bond. After that, all subsequent laminates bond to this first piece and each other on a molecular level, but applying them in a large-to-small order still maximizes the mating surfaces.