Plywood will always come up grainy without some good primer applied before paint application. See my videos on preparing a surface (specifically plywood) for painting.
Prepare Plywood for painting - CLICK ME
Duplicating the grain of wood requires an act from congress, (so to speak) Depending on what it is your doing you may not want to bury the awesome of wood like that, Paint over grain gives a nice factory style finish, When duplicated by artificial means, it ALWAYS looks phony, no matter how good one is at it, because at that point its ones interpretation of art through the eyes of the artist .... On sanded pine (like the main focus of this thread however) your probably going to be dealing with knots and sap on top of random grain patterns, so yes then its a compromise.
Why anyone would start out with pine plywood with a poor finish and then invest 40+ hours in building a cab is beyond me. Spend a few bucks more and get a better starting product. It will save you time in the end.
The finish on the outer layer of plywood is thin. Be careful not to over sand. Once you have gone through the layer, you are in a mess to fix things.
Below are a few of my thought. Sanding, prep and dry times are key.
My preferred method of painting is with automotive paint....but you need a big compressor (220v), a nice spray gun, and a place to paint. The finish comes out beautiful (after some practice)....and looks totally fake because it is too good. You are looking at $150+ in paint depending on the number of colors.(when I have access to good equipment). Sanding is key, but it depends on what you are spraying.
My second option is a super thin layer of drywall mud to fill the grain (don't need much), sand with 80 grit. Then roll with low nap roller using kiltz oil based primer, pre tinted may do two coats. Sand 120 grit. Then I spray with a crap compressor and hvlp gun. I spray Rustoleum (oil based) thinned about 25%. Sand with 250-320 grit (hand). Spray with a final pass and sand lightly with 800 (hand).
From here if I am to roll only, I use a low nap roller. Kiltz tinted oil based, sand 80-120. Rustoleum oil based, rolled with low nap. I usually do alot of "dry rolling" to make sure the roller doesn't leave marks. Sand wimmmmmRepea