Sand

[mikewb]

Also what grit sand paper do you guys use to make parts smooth befor primer?

[80stepsideguy]

I reccomend nothing finer then 320 before you prime.Your primer needs something to bite to in roughness wise if you go like 400 then thats to smooth and the primer wont adhere very well,320 is the finest i think  in grit.

thanks
pat

[JJSZABO]

I just finished an adult class on Auto Painting at our local Vo Tech and I was taught to sand bare metal with 80 grit first then 180 grit.  Wax and greese remove then spray primer.  If over existing finish, start with 180 grit then end with 320 and w&g remove and spray primer/sealer.

[Blazin]

What JJSZABO said is correct, but I would wash one or more times with wax and grease remover before you do any sanding. If not you can push the contaminants into the existing finish. You should also etch the bare metal with etching fluid or self etching primer before any other primer and top coat.
You don't have to sand existing finish with 180 before 320. he only existing finish I sand with 180 is around a repair area to blend in the the edge and take out the 80 grit scratches.