What would be a good sealant to use over Posca on rocks and plastic?

Question

I'm painting some rocks for a school project, little ones around a garden, and I'm looking for a good sealant/clearcoat. The rocks are pretty porous, but the paint is water-based, so it will easily come off.

I'm looking for a sealant/clearcoat that will work well. I'm also using the same paint markers on plastic. I sand the plastic, spray paint it white, color it, then clearcoat. I need a clearcoat that won't crack the paint like the one I had was. If there's one that would work on both surfaces, even better.

The Posca markers are water-based paint markers and the clearcoat was Rustoleum Satin Clear Enamel. I took an outlet cover, sanded it, spray painted it white, let it dry for about 20 hours, then painted it with the Poscas, let that dry for about 4 hours, and then used the clearcoat. I put a small fan on to dry, only for the clearcoat (is that maybe what made it crack?).

Answer 1

I'd use Krylon Fusion but I do the prep work, even though they claim you don't have to. I originally used it because I was told (by someone at a store that specialized in aquariums) that it's aquarium-safe once cured. It can stand being submerged in water for years and, not that you need this for your application, it won't leech nasty chemicals into the water over time. I looking for corroboration and found a good article on fishlab.com (https://fishlab.com/aquarium-safe-paint/)

I couldn't find any information on cure time (vs dry time) for the Rustoleum paint you used but I suspect it's much longer. Whenever I've looked up cure time for a paint, it has been somewhere close to a week. Given that you're switching paints between layers, you probably want to let your underlayers cure, then lightly sand them before using the marker paint.

Finally, do you know what plastic the outlet covers are? Some plastics require adhesion promoter before priming. HDPE requires flame treating before using adhesion promoter! If it's one of the difficult plastics, you can apply a sticker label to it and then paint that. Probably easier.