If I paint the bottom of my truck should I paint full strength with Rustoleum or should I do thin layers? The appearance isn't a factor but would a thick layer ever become fully dry?

Is there a way to scuff and wetsand a factory textured roof? I guess I wouldn't have to wetsand between layers but maybe the final layer would benefit from it. It may be difficult to buff but I guess a textured roof doesn't need much of a shine to it anyway? I don't want to lose the texture. It looks decent now but if the rest of the truck is painted it may not look as good. I may also want to change the roof color. Would it even be possible to get full coverage without a nappy roller cover? I'm thinking a foam roller might only put the paint on the high areas although it would flow but maybe it would pool up in the low areas?

How well is a thin layer of paint going to protect from rust? The truck has had 1 paint job in addition to the factory paint. I will probably have to sand at least down to the original paint and I'm sure some areas will be bare metal. If I paint the first layer and the truck sits for a week will the thin layer be enough to protect it from flash rust?