You know what they say....if a first you don't succeed!!!!
Had my work cut out for me on the plenum. As others have mentioned that the plenum surface is rough, and it is that....
So I am thinking if I just sand that one part down and re spray trying to blend in the area, it may look smoother than the rest of the plenum. So I decide to sand the whole plenum. Using 400 grit as suggested and mixing in with the 320 grit I had, took me a few hours to get it nice and smooth. Although all of the previous new paint did not get sanded away, as you will see in the pics. I am surmising that the rough area's were the high spots on the plenum or at least most of them. I did get a uniformed smoothness over the entire unit which was very acceptable. I was going to wait until tomorrow to paint, but that did not happen. Went into the open paint booth just awhile ago and ended up shooting about 4 total coats. I stayed with my previous methods and did not do a heavy color coat. I did not get over anxious either between coats, left enough drying time, as I did not want to produce any runs, etc.
I think it came out even better than before and that maybe because of the now smoother finish. I may or may not paint the numbers and the letters as I don't want to mess it up again....lol
sanding in process...

smooth surface as can be...

in the open paint booth...