There are products out there to do what you want to accomplish. When you are trying to smooth out a surface it goes like this
Filler. MANY thin (Key word is work with THIN COATS) layers of filler, use 120 grit to knock it down to shape, for the first shaping you can use down to 80 grit
More filler. Once you get your shape add a nice thin coat and sand it with 320 till it is smooth and the way you want it.
Glazing Putty. This is the key to all those small imperfections like air blisters that appear. Sand them down, knock off any extra loose bits and fill and sand..and sand...and sand.
Finally use a proper filler primer and sand with 600 grit using guide coats to find high and low spots
do a final prime and sand with 600 then clean the area and paint. If you do not intend to wet sand and polish the paint then wet sand your primer with 1200 grit to get the smoothest finish possible
They have what body shops called purple putty which is a type of glazing putty. In recent years they have created SPRAYABLE purple putty but you will need a proper paint gun to use it. If you really want to get the most out of your work go to hot rod websites that show body work tutorials. I grew up in a body shop so all of these products are old hat to me.
