Slick sand is a good product and will perform well under any type of paint top finish. Spray the coats out very wet and apply at least 3 good wet coats per application. Block the panel, and apply another set of coats if necessary. Be very careful not to over-catalyze the material, since it can set up in your gun!
I use an inexpensive Home depot gun to shoot primer with ever since I ruined a Devilbiss gun that I had owned for 20 years when some gelcoat set up in it on a hot day!
I usually get the panels perfect with 320 or 400 grit, shoot another coat, and then finish block with 600 grit before the finish coat is applied. Keep youself a can of black spray paint to mist over the primer to use as a guide coat. After just a couple of strokes of the sanding block, you will know exactly where all your high and low spots are at. The high spots will cause the black to dissapear instantly, while the low spots will keep the black paint in them until the panel is level.
Regards, John McGraw