Someone told me that I can laminate epoxy over polyester but not the reverse. Repairing a polyester nose bowl but need it to be stronger that the original so I was going to laminate glass cloth with West System epoxy to the back side of it in the high stress areas. Pretty important that it not delaminate.
The journey is the reward.
In building a Tango, I do some resin research from time to time. I found a boating link that discusses your issue. https://downeastboatforum.com/thread...tibility.1821/ . . . But none of the answers have a reference that go back any mfr's product data. I might ask West Systems about this. Or simply do a pull test on a sample coupon after it cures for 24 hours. Or for this one job maybe buy a polyester resin kit from an auto parts store. The important thing is to thoroughly scuff the base with 60-100 sandpaper. When in doubt, I always make a test coupon that I can test destructively. And I always save my mixing cups so I know the resin has properly hardened.
Thank you very much! Hadn't thought of asking a boat builder. And, after visiting that site, am impressed with how thorough they are. Much more complex than just which chemistry works with which.
Last edited by prasmussen; 02-10-2023 at 09:04 AM.
The journey is the reward.
I don't believe Epoxy resin will bond to a Polyester resin very well. Many polyester resins have a "waxy" finish to the surface. If you want to laminate glass using an epoxy resin (such as West System or Aeropoxy for example) over glass with polyester resins, you need to so some serious sanding through the resin finish into the glass cloth. That will give you the best "bite" into the old material. This would apply to something like a fairing, nose bowl, etc. Mixing resins in a structural application would, generally speaking, be unacceptable. Overall, Polyester resins are your worst choice for use in aircraft construction, but also your least expensive resin. Piper used it extensively in construction of tip tanks in the Cherokees.
The surface of any resin needs to be well-sanded before adding another layer if a resin surface is cured completely (24 hours). No type of resin has a "waxy" surface. If it did, no amount of sanding would ever get rid of it. But there are slick resin surfaces, and there are gel-coated surfaces, and there are wax-cured gel-coated surfaces. But there are no waxy resin surfaces. This is true of epoxy, vinyl ester, and polyester. I've used all three, and I've used them atop one another.
You can add another layer without sanding, but only if the base is not completely cured, and as long as it's the same kind of resin of course. If you change resin types, however, the base resin must be completely cured first. Just sand it really well. Or, if you know you're going to put another layer on someday, lay on a fabric called "peel ply" that is later pulled off to leave a rough surface that doesn't need to be sanded, even when the resin is completely cured. It's just Dacron polyester fabric that doesn't stick to resin very well. The weave leaves a rough surface that doesn't need to be sanded, even after the resin is completely cured. (Peel ply also happens to be the same fabric, exactly the same fabric, that the Stits process uses to cover fabric aircraft.)
Also, there are no "stronger" resins. All the strength of a fiberglass layup comes from the glass itself, not the resin. Resin by itself is fairly brittle and has no structural strength. The resin can be mixed with fillers to make things like a corner radius (because glass cloth weakens in sharp bends and needs a generous radius). When mixed right, it goes on like cake icing and you smooth it out. When it's cured, you sand it for the a wet layup that has the radius in it. These fillers can be stuff like microbeads and chopped glass. But fillers are never used in resin in making glass layups.
There are many good books on types glass, glass weaves, and resin systems. But seriously, nothing is better than an afternoon spent with a technical counselor who's build a glass airplane.
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