Wow, love how you were able to spray the polyester gel coat. I want to know how it adheres to the SMC and what you did to prepare the SMC for it to adhere. Because I would like to do the same to my wife's X2 while the engine is out. If you get a chance, please post up the specifics of your gun, and your air compressor. Also if you could recommend a faring filler for filling the deep scratches. Thank you for posting this thread. It is helpful to me and will be helpful to others.
It will take time to get a review of my methods effectiveness for the gel coat adhering on the SMC parts, but I will be sure to report back... I expect it might take a while to show any issues if it isn't quite right.
I'm not a chemist so I may be way off base... I had HARD time finding advice on coating SMC with gel coat. Every post I found just seems to come down to "it isn't worth the hassle, paint it" instead of telling a person how to do it. And well, I'm stubborn and have to learn the hard way I guess! So I took a lot of information from a lot of different sources in an attempt to create a method with a high chance of success. Boat forums, youtube, RC boat forums, corvette forums, ski forums, etc. I tried to verify every "fact" I found with more than one source. That said, here is what I did on the SMC:
I sanded the SMC parts down to the bare material. I filled any cracks and low spots I found with West System epoxy, and then sanded flat. (Any cracks were also epoxied on the back side with reinforcement material.) I cleaned the entire part with acetone. I used Rust-Oleum Filler Primer over the SMC and Epoxy in two coats, sanding after each coat had dried for 24 hours with 220. I finished with Rust-oleum primer sealer, in the hopes that the "sealer" would help prevent the leak-through of the SMC mold release agent that I read to be the main culprit with gelcoat adherence to SMC. Both these products are alkyd based, which I selected because I read that "An
alkyd is a
polyester resin modified by the addition of fatty acids and other components". Given that, I assumed that polyester would have a good chance of bonding to alkyd paint, and I had found some reading material that supports that conclusion. I used the TDS for the products to verify their base material was alkyd. There was also a Rust-Oleum Marine Topside Primer that was alkyd based, but I couldn't get it on short notice. Once the primer sealer was cured, I roughed it up a little with 220, but just a LITTLE. (I had also read that the mold release agent is brought to the surface of the SMC by heat, so excessive sanding was to be avoided.) Then I cleaned the parts with acetone again and sprayed gel the same as I did with all the fiberglass parts.
In terms of spraying the gel coat, I used an amazon HVLP gun with a 2.0mm nozzle.
Spray Gun I Used
I had a lot of help from some GREAT YouTube channels:
- Amazing video for applying gel by hand, with amazing details on how to cut and polish the cured product:
Refit and Sail gel coat video
- Great video for core concepts of gel coat as a coating and fairing material. By far the most valuable video I watched:
Fish Bump TV Secrets of gel coat
- SUPER useful video for understanding gun setup specifically:
The Composite Shop: HVLP Gun Setup
I have a 20 gallon compressor that does about 6cfm at 60PSI (the regulated pressure that I sprayed at). The gun uses about 7CFM, and so I restricted the flow to the gun slightly (bottom knob by the supply on my gun). If I took a 10 second break every 30 seconds of spraying, I always stayed at or above 60 psi. My compressor has a 25ft hose leading to a copper "air dryer" and a 25ft hose leading to my gun, which had the water separator on it as well. The first 25ft hose can be placed in a bucket of ice water if moisture to the guns separator looks excessive. It never did.
That is the details that I can recall. Tape/cover/mask anything that you value in the room that you're painting in... Using two filtered box fans for inlet and outlet air, and wetting the floor of my workspace seemed to keep the ambient air pretty clean and the space ventilated. If there are other specific questions, I can try to answer them as well.
