I am having a double brick and tile house built and will be painting the interior walls myself. The builders painters have left a lot of (ceiling white) overspray on the walls below the cornices and there is up to 100mil of undercoat on the walls aroud the door frames as well as up to 20 mil of oil based full gloss. My paint supplier said I must strip off all this before I can apply the sealer/binder to the plaster wall and if I do not do so the sealer will not penetrate the plaster. Therfore the top coats will peel away from the areas not primed.
When I asked the builder about removing this excess paint he said that the painters also use a primer and the palaster under the overspray etc. is effectively already primed and all I need to do is give the area a light sand before applying the sealer. Who do I believe?
If it is stuck now there is no need to strip off this primer and paint. To test the painted areas use a strip of masking tape; press the masking tape to the surface, rub the tape for full contact, then quickly remove or rip the tape from the surface. If excessive amounts of paint is removed, down to the raw plaster, then there could be a problem in the future.
Most likely you won't have any problems. Make sure to use a good primer on the plaster walls before painting. Also, a quick sanding on the over-spray areas is a good idea.