This is good for inside of cabinet.
Not nearly as hard to sand as Bondo.
Find it at paint store.

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Sand the inside with 80 grit to remove most of the paint. Then sand with 120 and finally 220. Prime. After prime, sand with 320 then paint.

I advise (strongly) that you heed the advice of BK.

Depends on the primer/paint and how heavy you go. Don’t go crazy just until nice and smooth and I wouldn’t go too thick with the paint. You want smooth and covered, grain should be visible when looked at closely as it absorbs and shows the paint differently but not overly noticeable and of course you shouldn’t be able to feel it.

You will never get rid of the woodgrain, no matter now much you sand. After the 220 sanding, it will be smooth as a baby's bottom, but there will be some minor woodgrain that you can see. The primer will take care of some of that. If you want it to look like a pane of glass, you can prime multiple times with sanding in between each coat, or use Bondo or another type of filler.

I rarely go for the pane of glass look as that's not how it looked originally.