What I think would be FAR superior would be to source an industrial urethane or acrylic enamel. It's catalyzed, single stage easy to spray.

Here's an example: A gallon of acrylic enamel, with hardener and reducer, just over $75 for the whole kit. 144 different colors. That's hard to beat. Equivalent to Delstar or Centari.

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Back to your question, yes you do need a good sanded substrate (original finish or primer) to spray any paint product over. No you do not need clear. Yes, you have to reduce it, probably with mineral spirits (see the label on can). There is a Valspar brand hardener that works with implement enamel which may help with durability. It's just that I would not expect a household uncatalyzed paint product like Rustoleum to hold up to much UV exposure. Yeah, yeah, I know you can roll it on and work with it on the cheap. For not all that much more, I'd just buy some real paint to spray.