Hi all, I am making some parts from Toray Prepreg Carbon. I am trying to find a good solution for UV protection. The parts will be left outside pretty much 24/7, and I am looking to get a year or so of protection without any yellowing.
Any thoughts or recommendations?
I’d buy a gel coat similar to this one. Proper gel coat application will provide the UV resistance you’re looking for
Automotive paint will provide great UV protection.
Oh yea paint…
I should have thought about that one, I just assume that everyone that works with carbon fiber wants to see the weave. 
Thanks, that looks good. Do you think there will be any issues with using gelcoat and prepreg? It’s cooking at 270 degrees for 2.5 hours.
I’ve never tried baking polyester but here are a few issues that might arise. If you’re using prepreg I bet that the polyester will completely cure long before the prepreg even starts to cure which might cause bonding issues. Also I’ve read that the melting point for polyester resin is around 200 degrees. It is one of the weaker thermoset polymers so that number sounds correct.
Any decent clear coat should supply uv protection
clear coat the part with 2k laquer after manufature that should keep it UV protected.
Never heard of anyone using a gel coat with a prepreg, let alone a polyester gel !!!
Epoxy surfacing coat with UV protection. You can buy it as a resin. Coat the mould…let cure or go tacky…apply pre-preg…cure…job done!
you can’t bake polyester resin or gel coat… it smells horrible and bad things could happen with the finished product.
If you need to bake the prepreg use epoxy surface coat, wether it be clear or colored. Try Resin Services for a clear bakeable epoxy resin surface coat. You will use a lot of 2K clear coat to burry the weave. Or if your part is just one or two, try brushing on clear epoxy after you bake the part and then demold it. Use a heat gun or butane torch to pop the bubbles in the epoxy, if brushing it on after the part is molded.
You don’t want to use polyester(gelcoat) directly on to epoxy parts. I’ve done it with boat hulls, but I’ve always used a skinlayer of vinylester in between.
Post curing polyester is perfectly possible(I always do it), but don’t go higher than 90 degrees celcius. Some vinylesters go up to 110 celcius. If you have a low temp prepreg you could use pe/ve moulds.
epoxy surface coat, nice stuff, but not for longtime protection. It still is epoxy, so it will get yellow and brittle anyway, but not as fast.
So what you do need : 2k paint. Any color is fine, if using transparent, go for one with an UV blocker. I use “de ijssel” from a dutch company, but Alwgrip is similar(not as good though 
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For a good finish on a male mould product, use peelply, sand it smooth, and paint! 
Use black pigmented (prepreg) resin, this will obscure the yellowing…