[wlesieutre]

Yep, sanding is an option. I've also seen people fill the valleys with some sort of goop filler and then start sanding from there, just to get it a little closer to smooth. But I can't say if introducing different materials like that might interfere with casting. They were doing it to paint over.

Another approach is vapor smoothing - you can use acetone vapor to smooth ABS prints. Acetone won't do anything to PLA though.

I've heard of other chemicals being available for smoothing PLA, but they're nastier than acetone and I don't know anyone who's actually tried them.

[msds]

Wax is a very good material for modifying/smoothing prints pre-casting, and doesn't mess with the casting process. UV-cure resin is also a good choice, as you can set it fully, sand it down, and apply a gel-coat for a shiny finish.

ABS is a very bad material for casting if you're planning on burning it out, but vapor smoothed parts do make very nice plaster molds if you can remove them. Chloroform works very well for PLA, but it is much much nastier.

[namibj]

You should be able to use acetone. See the solubility table [0] for PLA. It takes longer, but it works.

[0]: https://onlinelibrary.wiley.com/doi/pdf/10.1002/app.38833

[dezork]

Acetone really don't work well for PLA. You need tetrahydrofuran to smooth PLA in the same way that acetone smooths ABS.