20 MA project continues

[Blue_Heron]

I started to sand before the 2nd coat of primer and boy you can really see all the imperfections that you missed prior to the primeing.

Jorge,
I didn't see if you posted what brand of primer/paint you're using. Is it Awlgrip? Regardless, prep is going to be about the same for other brands.

150 grit paper is fine if you need to knock down some runs in the primer, but do the final sanding with 220 or 320. Anything coarser than that and you run the risk of sanding marks showing up in the top coat. And sanding of the final prime coat should be done by hand with a sanding block. If you spray on a guide coat, the final sanding should be more consistent. The top coat is going to show imperfections 100 times more than the primer, so go over it thoroughly on the final sanding.

From the Awlgrip application instructions:

The primed surface must be clean and dry. Achieving maximum gloss and distinction of image requires the primer be smooth sanded with 220-320 grit paper before topcoat application. Using a contrasting mist coat of lacquer primer as a "guide coat" is recommended. Smooth sanding until all the "guide coat" is removed indicates a texture free surface.

I think painting the bottom first is a good plan. It will show up any flaws in your prep work in a location that doesn't show. Then you'll have a better idea of how far to go on the rest of the prep where it matters.
Dave