I am painting a new aftermarket bumper for a Mazda 3. Color is a charcoal metallic.

The bumper was prepped by thorough washing with detergent and a gray scotchbrite pad. After this I scoured it again with scuffing paste and rinsed all of that off. After the water dried I cleaned it with OMNI wax and grease cleaner and allowed that to dry. Then prepped the entire bumper with SEM adhesion promoter.

After this had flashed dry I applied PPG Deltron 2000 base coat. Everything went well on the smaller areas of the bumper, but on the top flatter areas I was getting stripes in the metallic finish. (I am a relative beginner).

I am spraying with an Astro spray gun I purchased years ago to spray furniture lacquer finishes. Anyway I determined that I apparently had too much fluid being fed and after dialing this down and narrowing the fan pattern a bit I was able to lay down a pretty smooth coat. However...all my fiddling with spraying resulted in the base coat going on too thickly in one area, and a small sag developed.

The day is late and I can't touch it tomorrow due to work, so it will have to sit beyond the 24 hour clear coat window.

My plan is to wet sand the sag with probably 400 grit, then follow this with a light sanding of the entire bumper with 600 grit wet. After that spray on hopefully one more very light coat of base to try to even out the metallic pattern better than before.

Please tell me if this is the correct plan of action? Any other suggestions are appreciated. I am also considering buying a somewhat better gun, but would like to try to finish this small project with what I have.

The gun us an Astro Quantum gravity fed gun with a 1.3mm tip. My settings initially were about 20lbs pressure at the gun, fluid wide open, fan wide open. It was about 90 F at spray time, and the base was reduced with Tamco slow reducer 1:1

Personally, I think 400 is a bit too aggressive for base coat.
Not familiar with the base you are using but the tech sheet should have something on there about repairs.
The base I use says to sand with 1200 if I remember correctly.

I looked up the spec.s I had on that gun at one time. Try upping the pressure. The spec.s I have on it say 29 psi. I would think that would get you a "drier" more uniform coat compared with what you are getting now.

'68 Coronet R/T wrote:Personally, I think 400 is a bit too aggressive for base coat.

Agree. Coarser paper like P400 will tear the base, but may be necessary on the run. P1200, maybe even P1000 is a much better idea and should also be used to finish off any coarser scratches put in when taking out the run. A little spot of primer may be necessary here too.

Just as an aside, for OP, you did all the right things, just in the wrong order.

Aftermarket bars tend to be made from less than ideal plastic and are usually quite porous. The mould release oil gets in to the pores. Prepsol or W&G remover brings that oil to the surface. On a smooth surface that's good, but on a porous surface it can exacerbate the problem. For this reason it's now generally recommended that plastic be washed well and then treated with an alcohol based cleaner, rather than Prepsol. By using the Prepsol last you effectively undid the cleaning you'd done previously and the lines/stripes were probably crud that the Prepsol had left on the surface. A similar thing can happen if the Prepsol isn't properly removed or allowed enough time to fully flash off.

When you get a situation like this, don't hammer the paint on, thinking that will fix it. Apply a couple of really light coats over the problem area and allow to flash really well. That will give you a reasonable base for a light wet coat over the top to cover. It's the same procedure used if you get minor silicone fisheyes.