So I got these Red cranks and the previous owner sanded one of the arms down. What's the word on on UV exposure breaking down the epoxy? I don't mind clearing over the sanded arm to protect it but I don't know what to get to do that. Any suggestions are appreciated.

I don't either.

Look for a rattle can clear coat that states it has UV protection. Good luck and i hope you didn't pay much for the crank.

Why good luck? I don't see anything wrong with the crank other than it missing a logo on one side.

IME (in other applications) epoxy resin is pretty UV stable. You might phone someone who works with CF routinely to confirm. But do go ahead and paint or clear coat it to ensure that the surface is weathersealed.

Do yourself a favor. Do not go by a can of rustoleum, Duplicolor , etc...or other rattle can enamel / lacquer clear. They will not offer very good protection, or last long. If you must get a rattle can, see a body shop supply & get a 2k urethane can. Your other option is to stop by a body shop & ask them to clear it. As small as the cranks are, for a few bucks they can probably shoot them while / when finishing a job with left over material in the ***.

What about something I can brush on? Would be way easier

Epoxy is tough stuff -- I've used it to coat wooden canoes that I've dragged all over hell and gone. I suspect it might yellow a little with long-term exposure to UV, but any top coat you add will be mostly about aesthetics.

I suspect that you could find a brush-on finish of some type, but it would look like ass unless you planned to wet-sand it flat, then to rub it out to a luster that you liked. That's easy to do, just a bit of elbow grease.

You think a spar varnish would adhere to to the crank arm?

Go opaque black overspray.. being opaque is its own UV resistance for the underlaying stuff.

Or how about this?
SprayMax 3680061 Urethane Clear coat

I think I would clearcoat with a uv resistant coating. Probably an auto supply store would be the easiest source. If brushing is acceptable, look for a good quality urethane marine spar varnish. The reason for the coating is that epoxy, or some epoxy anyway, will absorb moisture.

I am not an expert, but I don't think any of the epoxies I have used would absorb water; again, I've used them to seal boats that are used in salt water. I don't know the formulation they use to make CF laminates, though, but I can't imagine it being much of an issue.

Keep in mind that what you're dealing with on the outside of that crank arm is epoxy (of some type). If I didn't like the flat look, I'd just polish the surface lightly until it got to where I liked it. I don't think you would affect the structural or long-term integrity in any way by adding a clear coat, just the appearance.

So far as the type of clear to use, you want to choose something that will stick to epoxy (I'm not sure if getting things to stick to epoxy is an issue, but it is for other bases, so be sure to check). I mostly spray lacquers, which I have sprayed over epoxy, but part of my standard finishing steps include spraying an isolator coat that anything sticks to (complete overkill here!). Again, I don't know how well things stick to epoxy directly -- it might be fine.

As suggested above, an auto paint-supply shop would have good answers for you and all the materials you could ever imagine.

Thx for the suggestions. I don't mind trying the easiest/cheapest solution first, if it doesn't work I can always sand it off and try something else. I just want to make sure that whatever I do use won't degrade the resin in the carbon and weaken the arm.

Epoxy is usually waterproof, but urethane resin is not ?? That is what we often call "fiberglass".....they use glass strands and urethane resin....and for a boat or other outdoor item they use a "gelcoat" that is waterproof.

No idea what they use in "carbon fiber" though, it is an epoxy, or if they use urethane resin.

I had always (wrongly" assumed that the "fiberglass resin" was waterproof, til I did some digging into it and found out that it is NOT waterproof :-).

Bill

Where's a materials scientist when we need one?

My GUESS is that whatever is holding that arm together is tough stuff and probably pretty darned immune to an occasional water bath. I'd also think that buying a spray can worth of most any clear finish that is available locally that seems even mildly suited for the job would be pretty safe.

I'm kinda thinking the same same thing. In fact, a local carbon repair guy just told me that carbon fiber doesn't need to UV protected for whatever that's worth.

You'll want the latest kit upgrade soon enough, and make it all someone else's problem ..

FG Resin is More a Polyester as would be the Gel coat [1st layer sprayed in the mold to begin layup ,


the clear coat paint mentioned, a urethane would go sprayed on the finished part..

which is what the OP has ..

Dude seriously I can never understand what you're saying...

Meh

Can you try explaining what you were saying in a different way please? I really am curious to know what you were saying.

Just sayin' Logically, Opaque = cant see thru it (by definition)

it will block all light . so opaque black paint, will protect the underlying Composite by not passing light thru it .

the stuff, the pigment (black would be carbon)and the vehicle that makes it Liquid . that vehicle is also selectable for its UV resistance.

That will work for you.