Rustoleum is an oil based enamel that absorbs oxygen from the air for the curing process. The hardners or catalysts cure by an entirely different chemistry. It sounds like the hardener is not soluble in the Rustoleum. It may be sticky for a long time. I think I'd use stripper or thinner to remove it. I don't know the exact chemistry of the hardner but it's very unlikely it will do anything with the mineral spirits. It may not be soluble in the mineral spirits so you end up with "spots". Mineral spirits is a perfectly good thinner for Rustoleum, I've used it many times spraying things like outdoor furniture.
