I am painting a child’s room and painted part of a wall to see the color…however I didn’t prime it first. What do I do now? Do I have to use primer first?

My siding was painted a while ago and looks good–but, I noticed a couple light bleeds onto the brick this year. Oxidation, a friend said. Is their a clear sealer that I can put on now to help get more years out of paint? Thanks for any help.