let the contractor do what he does, that is why you are paying him(her)

Thank you, Chris. He is a general contractor who is doing remodeling for my house, not a professional painter. He said he could also paint the house interior for me.

I , then, would be looking for someone who actually paints,

like a painter

GC's are NO good at it( or much of anything else in my experience)

Well, some do it all types are actually not bad painters, but this is very, very rare. Anyway, whether or not you spray or not really doesn't matter. A pro uses the sprayer to get the paint on faster. Whether or not it is sprayed, the actual paint gets rolled no matter what. We call this backrolling, meaning application of the paint with a sprayer, then physically rolling the paint while it is still wet. If you just spray it on and leave it, it's fairly likely that the paint will dry uneven and look crappy. The roller helps to even everything out. The only time I would spray walls or ceilings and not backroll is if there was a heavy texture or popcorn, which would disguise the uneven look you may get on smooth drywall.

Chris, thank you very much for your help.

Paintguy, really appreciate your detailed information, which is very helpful.

Another question. The contractor sprayed semigloss white paint on the door casings and by accident their adjacent walls. To paint all the walls with flat paint of different light color, does he need to apply primer to the wall with semigloss paint first before applying the flat paint?

Thank you very much.

either that or just sand it
it does not need to be sanded off, just scuffed up