It all depends on what shape the panel is, but heres a quick walkthrough; If the panel needs body work...
1)first of all, all body filler (aka bondo) must be applied to bare metal.period.
that means that you have to grind off (36 or 40 grit is ok) the paint before body filler or filler glaze is applied.
2) block the body filler using a long board and 80grit paper (again, use guide coat to make sure you have no low spots in your work)
3)after bodyfiller work is done, DA the sorrounding area of your work with 240 grit, then feather edge the 240 with some 320 grit
4)if the rest of that panel doesnt need further work; just scuff the rest of the panel with red scotch-brite
5)NOW apply your primer; NOTE that some paint companies (dupont for example) recommends you use etching primer on bare metal areas, then to use primer filler on top of that, while others like sherwin williams go straight to primer filler.
6)when spraying your primer, spray it only a few inches over the work area.The whole panel doesnt need to be primered unless deep scratches need to be filled in.
apply 3 to 4 medium-wet coats. This is because you want to make sure all the deep scratches get filled in, and because alot of that primer will get blocked off so you want some breathing room rather than having to reprimer.
read instructions on your primer, 4hrs is about average dry time before you can dry sand
7) mist some black guide coat over primer, and using a durablock sanding block and 320 paper, block off your work area in long smooth strokes, use horizontal, slight diagonal and some vertical strokes to make sure your panel is straight.
8) now using a more flexible wet sanding block, wet sand your primer. Make sure you get a nice fether edge where your primer ends, any hard lines will shop up in your work if you dont, or much worse, cause fish eyes and your paint to crack.
9)after all the guide coat is went sanded off, you sholuld be ready to use a clean shop towel with wax&grease remover to clean off the panel, then use a tack cloth to wipe of all dust, and are now ready to spray your sealer, color, and clear.
If no body work is needed, but there are deep scratches or imperfections in the paint..
1) DA imperfections with 400 paper
2)scotchbrite the rest of the panel
3)wipe clean, tack off
4)seal, color, clear.
If no work is needed at all..
1)scotchbrite
2)wipe clean, tack off... you know the rest
Single stage paint is what youre refering to, it doesnt need clear because it can be buffed to a shine.
Base coat/clear coat paint however is whats used on any descent shop and what cars come with from the factory. The base coat is the actual color, and the clear is applied after the base coat is allowed to flash (semi dry). Its higher gloss and lasts longer.
remember, 90% will depend on the prep work, 10% will be actual spraying of color/clear
hope that helps.