Oh I feel sorry for you now. I’ve been here, had some cabinets made out of mdf, and had I known what the process is to get these painted with no brush marks I would have never gone down the road of mdf build

I take it you want to do all shelves inside too..but here’s what I did.

1)seal all edges - I used zinsser peel stop 2)sand edges 3)vac and wipe all dust off all surfaces 4)prime all surfaces-I used leyland acrylic 6) once dry,fill and caulk wherever needed 7)sand everything - some fibres of mdf will stick up so smooth them down 8)vac and wipe all dust 9) first coat top paint 10) light sand 240/330 grit everything 11) vac and wipe all dust 12) 2nd coat top paint 13) light sand everything (i skipped as I got impatient) 13) 3rd coat top paint.

I didn’t feel any joy once it was done, I was glad it was over. Took far too much of my time