Bob,
My knowledge of polyester / vinylester resin is based upon what I've read, and my concerns regarding adhesion will defer to experience; however, I suppose success depends upon the loading encountered.
I trust you, but my experience is different. My knowledge of epoxy is based upon roughly 20 years of experience which includes building a 15' plywood and epoxy skiff and numerous repairs and projects. I have never seen epoxy shrink except when exposed to UV, and that shrinkage/deterioration can be significant. I have filled numerous seams and holes with filled epoxy, sanded and painted them and they are not detectible. Many times people have accused my plywood boat of being made of fiberglass. I faired the plywood hull with epoxy and painted it with polyurethane paint. It is now over 10 years old.
Vincent,
I'm not trying to 'one up' you but my perspective may be unique on this forum. It goes back to building production fiberglass boats as well as FG surfboards and mooring bouys in the early 1950's, formulating epoxy adhesives in the late 50's to early 70's, undergrad and graduate research on plastics and resins, 42 years in plastics research and forensic analysis of plastic failures all capped with better than 40 years of fiberglass boat ownership and the asociated structural and blister repairs they have needed. And, yes I've fixed stuff on the wood boats that preceeded the glass ones.
That said, resin selection AND repair technique are critical to any job. Epoxys have inherently better adhesion to fiberglass than do polyesters. Vinylesters are almost as good as epoxies and are my preference for many things because they are easier to work and have lower moisture absorption. And by the way, vinylester resins are made from epoxy resins and share many characteristics like toughness. They are also my choice for barrier coats and the only repair facilities that I am aware of on the Bay that will provide a guarantee on a blister repair job are using vinylesters because of the adhesion and lower moisture absorption. And BTW, the new hulls with blister warranties are using vinylester at least for the outer part of the laminate schedule.
But as in all things, procedure and technique are paramount along with material selection to getting a long term satisfactory repair. Many a boat has had it's hull cut open to remove a leaking fuel tank. The repair is typically a vinyl ester 'patch', but done right (scarf angles etc) the repairs hold. At least I'm not aware of a failure, even among off shore cruising boats like the Defevers that roam the seas. Personally, I'd be nervous, but the success seems to speak for itself.
As far as shrinkage goes, all thermosetting resins shrink! Period! It is simply inherrent in the chemistry of the curing or hardening process. Think of it as the opposite to water freezing and the expansion that occurs which can break pipes etc. The amount of shrinkage in a curing thermosetting resin is a function of resin type, filler content and type, size of the hole (as in this discussion) and how warm the patch material gets when it sets up. But the shrinkage sets up some pretty significant tensile stresses which over time can lead to a debonding when a putty is put into a rigid structure like a hole in thick laminate. The smaller holes and prewetting the laminate can help. But I will always grind back and fair with wetted glass mat or fabric depending on the scope of the repair. This changes the stress type from tensile to shear and the shear stress in laymans terms is lower and more spread out. Simply, it makes for a better, less stressed bond.
Nuff said. If I were making the repair and the area would be visible after painting, I'd grind back and fair in glass fibers. This from experience. Now.... the first manhattan of the day awaits.
Dr. Bob
Still waiting for my tranny repair
