if they are textured they need sanding down to remove the texture, if they are smooth you can sand them with 320 grit and then around 600-800 grit, clean them off and prime them, 3 thin coats of primer leaving around 15-20 mins between each coat, leave to dry for a good few hours, wet sand with 800-1000 grit wet and dry, be careful not to rub through the primer, if you do you will need to reprime them, clean them off and dry them, 3-4 thin coats to paint, again 15-20 mins between coats, about 20-25 mins after the last coat of paint you can lacquer, 3-4 thin coats again leaving 15-20 mins between coats sanding between coats is just a waste of time, it pretty much has no effect on the final outcome, when i was doing my qualifications in college, as part of our coursework we had to do like an experiement, 4 panels were painted, 2 with rattle cans, 2 with spray gun. then one of each was sanded between coats of primer and paint and the other 2 not, and there was no great difference in the final product so its just a waste of time and effort the only need to sand between coats is if you've got runs in your basecoat/colour coats, if this happens, let the paint dry for a few hours, wet sand with 1000 and then redo a few coats of colour