I have done a lot of painting regarding ABS plastic, and composites such as fiberglass. If you are going to rattle can it, it will always look sub par, even if you are really good at it. If you want a good job you will have to use an actual paint gun but I doubt this is the avenue you are going since you are suggesting truck bed liner.
regardless of you application the prep is where the money is at. You will need to clean everything up, fill any scratches or scuffs and do a step wise wet sand from 300 -> 1000 grit. I recomend doing a primer check to ensure you have low spots and any trouble areas marked so you can re-sand them and move on. Desired piece, clean with acetone, scuff up for bondo or what ever fix/prep you decide, apply, sand down to rough finish, apply a base primer (opposite color from the body work), then sand that down in a progressive fashion with finer grit (wet sand with a sanding block, I prefer a semi rigid type for plastics/composites) and then clean again, clean with a honey cloth or something else to remove any fibers and if you want it to look good you will need a proper paint booth or some place with no flying particles, once your level of base coat has been set up I would sand with 1000 grit to remove orange peel and then apply the clear coat. This is the very basic method.
Even if I rattle can I still prep, primer, wet sand and paint then clean up orange peel and clear coat.