Hi - total beginner here. The previous owner left behind these already painted or primed plain bifold closet doors with (I think?) veneer surface. They have lots of little bumps and some wood grain. I'm trying to get the surface as smooth as possible to paint with SW Agreeable Grey in satin and add some modern square bar pulls. On thrse particular doors (I have a bazillion I'll be doing) the wood grain isn't so pronounced, but there are some little long lines. Should I fill those with Aquacoat white cabinet wood grain filler...?

And the builder issue is how visible the bumps really are. What I've tried so far with these doors and another sample old door elsewhere:

- I sanded the doors with 220 then a bit of 100 and an orbital sander (I don't have any grit between those on hand) and managed to achieve some smooth areas but other areas just remained bumpy - did I go too far with sanding or not far enough? Should I try to continue sanding without going all the way through the veneer?

- On these planks, I used a 3/8 nap. Is my nap making it worse? Would a second coat of the SW grey and/or a different nap smooth it out? Is my random technique of rolling roller upwards and downwards the problem? Should it just go one direction?

- On some scrap wood I tried Rustoleum filler primer (an auto spray primer.). While it smoothed out bumps, it was a smelly, messy job and the first coat of the SW paint over the top had a metallic sheen. Should I just use the Rustoleum primer, then cover with another regular white primer then the SW grey paint? Is there another high-build primer (preferably white) I should try?

I'm very confused at this point. (Also, yes, I'll wait for the wood filler to dry then sand and wipe dust before painting those areas and I'll likely hit any bare stripped-back areas with a bit of white primer before painting. And the edges of the door look raised/ridged and need more sanding, yes?

Basically, how smooth is it feasible to get this door and what's the least painful way to do it? Thank you for any pointers.