Start with a high quality sandable primer/filler. The primer acts as a middle ground between the Bondo and the paint, increasing adhesion of the paint. The filler part of primer/filler will actually fill in tiny pinholes that would otherwise affect the quality of your finish. Apply a coat or two of primer/filler, then resand the area. When you're satisfied with the finish, you're ready for real paint.
Apply several thin coats of paint rather than one or two thick layers. Always moving the can in one direction (left to right, or up and down), this will minimize runs and sags in the paint. Less paint and thinner coats is much better than thick coats. Also be sure you let your paint dry between coats or you may get runs or sags. Once the paint is thoroughly dry you will want to remove the tape and do a very fine wet sand (600 grit paper) on the whole area. This will blend the new paint with the old and remove any lines left by the tape.
