I have found that the roller leaves too many lines, so I have used a sponge, a simple, small-cells, kitchen sponge from the dollar store, 10 sponges for a buck.
Lightly sand the old paint, dip the sponge in paint and I tap the sponge gently on the panels. Works great! I have done a '70 Impala in flat black, excellent finish, you do not even have to sand the flat finish! Usually 2 or 3 coats cover well.
Then throw away the sponge.
