remove them first to make life easier!
a nice tug on the lower grill in any corner will help it come out. the top grill is a bit trickier as you have to remove the lock.
wash the grills in fairy liquid and make sure they are dry and then sand them down and wash again before attempting to paint. remove all contaminents and paint in a warn environment to help adhesion.
i think lenny/tdcist/gforce have guides on how to remove the grills.