Thinned matt white emulsion paint would act as a primer, if you want to go down that road, add a very small amount of black, so when you put the 'Top' coat you can see where you have been, white is a hard color to see when applying more coats. Using thick
un-thinned paint (should be thinned anyway normally add 5-10% water, but instructions will be on the tin) will just pick up the flakey bits if you don't give it a quick sanding.