I recently moved out of my apartment. I had painted the bedroom a light shade of green that could easily be covered with 1 layer of paint by the apartments after I moved out. They are charging me the cost of repainting because they said they had to use a primer since I had used a semi-gloss paint and they use flat. Is this true?
A primer is only needed if you used an oil base and they use latex. Latex won't stick to oil without a primer.
It is possible they needed two coats because the first coat slid around a little when applied over the slick semi-gloss, this is also called a prime coat. This would cause insufficient coverage on the first coat. It wouldn't affect adhesion of the first coat.
If the painter who paints the wall knows what he's doing, a primer is really not required… but careful rolling is required and to be sure a "wash coat" using the finish coat paint, could be applied first, which would act as a primer… then when dry (1hr) a finish coat carefully rolled will do the trick… all of this assumes that a High Quality Paint is being used (often not the case with apt repainting)…