I asked Don Casey the question directly and got a very comprehensive logical reply. The answer is that biggest first is best but lots of people have done it the other way for a long time and it has worked OK.
As a point of reference I just watched the http://www.bennettmarine.com/store/s....php?cat_id=59
Bennett Marine video and they show the expert using a patch with all layers the same size “large” and just slapping it in. It left a lump around the outside edge that he just ground off.
So it looks like we now have Don Casey, West Systems and Bennett all saying about the same thing.
Also this minor flap as shown how forgiving fiberglass is as a material for repair.
>>> Don Casey Response
Your original question:
Assuming a 3" hole in a 1/4" fiberglass skin ground to a 9" depression a West video shows the 8.5" piece of repair glass being the first piece laid down while your books show the 4" piece as first and 8.5" as last.
Do you think it matters? Would one way be better in some circumstances than others?
Also some people say patches on a glass hull can use epoxy while others say it is better to use the original resin to match the shrinkage and flex of the original material and not create a hard spot. Who is correct?
Our expert's answer:
When filling a depression 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 only attach to the original material at the perimeter—in effect a butt joint. Laying the largest piece first maximizes 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.
In addition, when using polyester or vinylester resin, the first piece must be mat. If it is the largest piece it gives you the required mat base for all subsequent laminates. If it is the smallest piece, even if it is mat, 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 makes less difference to the strength of the repair. Small first tends to yield a neater appearance but the void created where the larger layer overlaps the edge of the one beneath can create a void that lets the thinner epoxy resin drain through unless you thicken it. A large-to-small order avoids this. Also the relatively recent development of epoxy-compatible mat makes a large-to-small schedule using mat between the cloth layers a possibility.
Bottom line is that despite counsel to the contrary in a number of books with my name on them, large to small is usually better unless you have a specific reason to do otherwise. A new edition of This Old Boat out late next year will reflect this change in thinking and technology.
As for which resin to use, I think the better adhesion of epoxy trumps all concerns about differing flex characteristics. Here again, however, you are right to question the “experts.” The fact is that some fairly recent testing show a secondary epoxy bond just 15 percent stronger than polyester. If you were thinking it was more than that, this might influence your choice of resins based on either cost or consistency with the original lay-up.
>>> >>> End Don Casey Response