I used Rustoleum Enamel and rolled it on with a small foam roller i bought at walmart on the outside of my hard top. I roughed up the surface with sand paper i dont remember what grit but it was probably around 200. It has held up very well and very little has come off just very small chips here and there. It has faded but it dont look bad.
I know a lot of guys use herculiner on the outside of the top because it is textured. The problem is that it is way to textured and when you get mud and dirt on it you will pretty much never get it all off and it will look like a bag of smashed assholes when the sun fades it. I know people who have herculined whole trucks and they look like shit.
You could probably get away with something like Monstaliner though because its not nearly as textured. I know a guy who did his jeep in Monstaliner a few years ago and it still looks good because you can actually wash it and it holds up to UV light. But if you just want something cheap and easy that works use Rustoleum and a little foam roller from walmart.
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