depends what it is and how big. ive had success with sanding, buts it been on flat surfaces with thick walls

I'm going through this ATM, not finished yet

So far, hand sand with 400 grit W&D, plastic primer (dunno if it needs it but I had some) then a couple of coats of hi build primer aerosol.

That's as far as I got last night, will sand the hi build tonight and then paint it black and see what it looks like

If you can print ABS it can be smoothed by brushing on acetone. Otherwise it’s jusr work and filler primer.

PLA is the best filament for sanding and painting , i have had the best results wet sanding to minimise the heat and you can usually get a really good surface finish