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Full Version: Can You Use Poly on Polyiso??
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I am reading conflicting reports on this. Everyone seems to agree you can use Epoxy resin. I know they make pre-fabbed foam/fiberglass panels for the construction industry. So can you glass polyiso foam using polyester resin and has anyone done it? Results?
Polystyrene is attacked by polyester resin but not polyurethane.
Epoxy is the ticket.

Why it is not generally used is it is more expensive, and requires it be mixed thoroughly in a precise ratio with not much room for error.

Polyester resins, usually called fiberglass resin( and erronioniously in my opinionn , only require an evil toxic catalyst, and if under catalyzed, simply cure slower but will eventually harden.

Epoxy hardener is pretty toxic too though, but at least tools can be cleaned with vinegar, and once cured the epoxy dust is inert.

Here is a link to some pretty good fairly inexpensive epoxy that I use on many projects, not just surfboards:

A simple 2:1 mixing ratio by volume makes working with epoxy much simpler, but I prefer weighing it, and this is 100:45 on this specific epoxy.

I've little experience with other epoxy formulations other than Resin research and System 3 resins, but most are pretty good if designed for boats/ marine applications. Some of the bartop epoxies are too thick to wet out fiberglass cloth and lack the flexibility one might require. I've used West systems, long ago. Not personally impressed, but they are otherwise highly regarded and easily available, but IMO, avoid and seek others

Be sure to have a mixing stick that can get into the corners of the mixing cup, do not spill any down the sides of the cup, mix it twice as long as one believes they need to, and be sure to use gloves and keep uncured epoxy and epoxy sanding dust off your skin When mixing scrape the sides fo the cup and all 5 sides of the mixing stick more than once.
Some people are highely sensitive to epoxy hardener, those not sensitive to it, can get sensitized to it with cumulative exposure.

Epoxy is stronger, and lighter, and more flexible, and bonds stronger to most every surface, but polyester resins can generally handle heat and UV light better.

Polyester resin requires a surfacing agent in it to be able to be sanded. This resin is called sanding resin or finishing resin or perhaps gloss resin. Without this wax/ surfacing agent, it is called laminating resin, and will remain tacky and will gum up sandpaper, but this tacky surface is great for secondary bonding. Secondary bonding to sanding/ finishing/ gloss resin requires the surface be cleaned of the wax and scratched with sandpaper, for mechanical tooth. 120 grit is a good number for scratching, and new sandpaper is better than old for secondary bonding as the mountains and valleys are sharp which increases surface area for the bond.

While many backyard surfboard makers will buy insulation boards from home stores to make thier own surfboard blanks, they either have to use Epoxy resin, or they need to 'Spackle' the shaped blank with another product to prevent the solvents in polyester resin from eating the foam. I am not up to date on these 'spackling' products that they use but long ago when an EPS or XPS surfboard, that was glassed with epoxy needed a repair, and all that was available was polyester resins, we sucessfully covered the foam with elmers glue, then after that cured, fiberglassed over the damaged foam. The bond strength seemed OK, but it is not as if we picked and prodded and tried to make the fiberglass shell fail.

System 3 SB112 epoxy resin is said to be a 'barrier coat' POlyester resin WILL cure atop of AND bond to this
cured epoxy, which cannot be said of all other epoxy. I've had a drop of catalyzed polyester resin refuse to cure atop epoxy, and I've also had it cure atop of it, but not bond to it, and I've also had it both bond to and cure atop epoxy.

Getting it wrong is usually much more work than having tested for compatibility on a small portion first. Do not assume any spackle or barrier coat and polyester resin epoxy resin combo is going to work. Some weird stuff goes on on that chemical level and it is best to not take chances when the posibility of a failure means much lost time labor and material.
Thank you both for your reply. I guess my point is if polyiso is a cheap yet functional foam for composites using polyester resin then that is a game changer price wise. I priced the urethane/epoxy materials and damn are they expensive for a larger project. Another concern I would have is I read polyiso shrinks over time which would possibly lend itself to delamination making it a deal breaker even with epoxy resin. Maybe not.I want to look at the industrial side and try and figure out what they are doing. I know they sell 4X8 and larger "sandwiches". I miss the days when you could get the materials to make a surfboard and get change back from a $10 bill.