might be a stupid question but....

3 months ago I spray painted the truck using Rustoleum black gloss...but a week later the paint ended up getting a flat tone to it and turned out horrible.........2 days ago I sanded it down a bit and repainted it using a roller this time (using Rustoleum oil-based gloss) as you can see in the picture (the bed door and rear bumper only) it looks kind of shiny...
how do i keep it shiny? and avoid it getting that flat tone to it like the 1st time....
thanks in advanced

3 months ago I spray painted the truck using Rustoleum black gloss...but a week later the paint ended up getting a flat tone to it and turned out horrible.........2 days ago I sanded it down a bit and repainted it using a roller this time (using Rustoleum oil-based gloss) as you can see in the picture (the bed door and rear bumper only) it looks kind of shiny...
how do i keep it shiny? and avoid it getting that flat tone to it like the 1st time....
thanks in advanced
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