The way that I have done the dashes in is if you have a texture in the dash I take out the dash then take the sander to it for a min or so to take off soem of the gain in the dash. Then I break out the paint gun with some high build primer and shoot it. Then I sand some more after normaly 2 shooting of primer it is smooth and ready for paint. I prep it the same way as I would before painting a car or anything.