Painting without primer?

Would primer still be advisable for unpainted surfaces? Purchased an aftermarket replacement bumper that is plain black. Going to hit it with duplicolor mgm and clear coat

Wondering if I need primer if there is no paint on it to begin with for this rear plastic third gen bumper.

 

Is this a front (plastic) or rear (metal) bumper we're talking here?

 

It is a 3rd gen rear plastic bumper.

 

Ah, I see. Typically for plastic you'll need to first scuff it, followed by spraying it with adhesion promoter, primer, base coat, then clear.

 

Would you recommend a specific grit sandpaper to use?

 

I think Scotch-Brite pads would be better suited. What color (grit) pad you use will depend on what part of the paint process you're doing (i.e. initial plastic scuffing, primer scuffing, etc.). All kinds of info out there on 3M Scoth-Brite pads.

 

I should've added, typically you'll see the red/maroon and grey pads being used.

 

Agreed.

Here's a test for determining what you have on the piece you purchased. Use a very light grit, 400 or finer. LIGHTLY scuff the surface for a few seconds. If you get a dust that comes off it is already primed. If you don't get any dust, then it is what is called raw plastic, and an adhesion promoter will be required.

 

400 grit, then 800 grit, then 1200 throughout the process

 

While mostly right for general purposes, 400 is too fine a grit for surfacer/primer to adhere. The jump in between those grits will also leave scratches behind that cannot be removed by the even finer grits.

 

Usually type of plastic is stamped on back of part . Some plastics need different adhesion promoter . With that bumper piece thinking regular plastic adhesion would work . Scotch brite pads do seem to work better .