What's the difference between epoxy and polyester laminating resins? I used a lighter polyester resin to penetrate chop mat.
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Well for chop mat polyester is supposed to be much better than epoxy but I'm not a fan of chopped mat for plywood tanks. Don't get me wrong, I have used it on plywood tanks that held water but I'd much rather use medium weight cloth. Mat is the weakest of the three types of fiberglass reinforcement, isn't really of much benefit on flat surfaces and also takes the most resin to wet out, which overcomes the fact that it's the cheapest. There are some in the boat building community that also feel mat is also the mostly likely to lead to water penetration if not applied properly.
Now assuming one isn't using fiberglass mat in a plywood tank build, the comparisons between epoxy and polyester are a bit more straightforward.
Polyester tends to flash off quicker and sag less, which in some ways makes it easier for vertical or overhead laminations. It is also less expensive but the price difference between polyester and epoxy seems to be shrinking.
The downsides are that is has a strong odor, is more moisture permeable than epoxy and is also weaker and less elastic than epoxy.
There are those like the previous poster who say polyester is toxic when cured. I'm not up to date on that but there are recent builds here on MFK that were sealed with nothing buy polyester and have held water (and fish) for years. I personally have built polyester sealed tanks that held fish for decades but to be fair that was a long time ago. I can't say for sure modern polyester is still safe.
As far as epoxy, it is generally stronger, more flexible and has much better bonding properties than polyester. It also exhibits less shrinkage than polyester.
When used on cloth on horizontal surfaces many find it easier to work with since it cures slower and flows out a bit better. Some beginners like epoxy better because they generally feel like they have more time. Of course epoxy cure rate is much more susceptible to temperature so even a 10* temperature difference can be very noticeable when wetting out fiberglass. On the same hand, epoxy is probably more susceptible to mixing errors than polyester, although you can read forum posts about people having trouble getting both types to cure properly. This is usually due to incomplete mixing or in some cases incorrect ratios of resin to hardener.
One other point, my comments above only apply to the thinner, less viscous types of epoxy. I have zero experience with the thicker types of epoxy that are trowled on.
I hope that helps. I'll write more if I think of something.