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.