If they are small imperfections then a tube of Knifing putty filler from a car supplies shop is the best. Use sparingly and when dry sand smooth with 1500 or higher wet and dry paper. You may need to treat larger spots more than once so take your time. A light spray with primer will show any remaining imperfections that may disappear after a further light sanding with wet and dry. Like any painting a good preparation will give the best results and usually takes some time.