You shouldn't need to do anything, 3D plastics are stable with no health risks unless you melt them. And paint will only make glueing things inside to it harder since you'll be glueing to the paint layer instead of the plastic itself. So it's mostly just a cosmetic choice for you. if you just want it smooth, you could try rotocasting some of that 3d smoothing resin in there, but I don't know anyone who's tried that.
[USER=9318]MinuteFett[/USER] Yeah that's definitely an issue with PLA, and lower melting casting plastics too like Smoothon 65D. Hopefully OP's was printed thick enough to make it less an issue. Something like this should really be printed in PETG or another higher melt plastic.
