Whereabouts are these particular trims you speak of?
I would start with that grade paper, however each time you apply primer, I would give it a higher grade light sand, probably a 600 or 800 just to give it a bit of a key, then degrease and clean with a washing detergent, and repeat spraying.
With the paint you can do the same process, however going too hard with the sandpaper may mean the primer will show through, which could lead to an imbalance in colour in coats.
There are various primer colours which compliment different top coat colours. If you're spraying a more vibrant, lighter colour, use white primer.
Also, make sure if you're using an acrylic based paint, you use a suitable lacquer. I'm sure you know all this, but putting it out there just in case
