I would like to use nail polish to paint and smooth a PLA model. Could nail polish damage a model made out of pure PLA?
This is an ugly question, and an ugly answer.
Nail polish doesn't come with an ingredient list. Nor are there MSDS's available to refer to what solvents are in it. So it's "unknown 1"
Most every filament company will not give you a list of compounds for colorants or plasticizers used. MSDS is off the table. So, even though we do know what PLA and ABS is inherently, we have no clue about the other ingredients used to make your particular filament. This is "unknown 2".
What both of these comes down to is a very nasty situation indeed. We know for a fact that ABS dissolves into acetone. PLA does not.... Except we have had reports of PLA that does. When we look further into those responses, and ask them to do a burn test, they smell popcorn and something acrid (ABS).
This guarantees unless we have virgin material and MSDS in hand, you only have a close guess at what material you have. And that's to say nothing about "nail polish", and its components.
There are also other solvents that will dissolve PLA. MakerBot back before they turned evil, noted a solution called Beta Solution (90% isopropyl alcohol, 9% KOH, 1% Al(OH)3 ). This stuff is noted for dissolving PLA and leaving ABS intact.
Long story short: "Could Nail Polish damage pure PLA?" We can't guarantee purity of PLA, we can't guarantee contents of nail polish's solvents. The only answer is to print a swatch and test yourself. Calibration cubes are quick and easy.
Nail polish will not damage your PLA model.
1) Let's talk about "gel" nail polish. Gel polish self-levels, does not drip, is cheap, is much more health-friendly than many finishing methods, and only takes 30 seconds under a suitable strength UV/LED lamp to dry, providing a thick smooth coat (clear, color, glitter!, anything) that makes any layer lines disappear.
2) The FDA regulates nail polish in its "cosmetics" category, so, not only are the ingredients known, they have to be printed on the bottle.
3) Gel polish consists of various methacrylate monomers which undergo a radical (in the chemistry sense, although it is pretty darn cool too) polymerization process that is UV-activable. Filament is primarily polyactide, which is polymerized at room temperature. Polymerized polyactide reacts with methylacrylate at the ends of the long strands (not in the middle, which is what you would describe as "damage"). Methylacrylate actually makes the PLA ends less reactive (read: it makes it stronger). As an industrial material, this compound is known as "supertoughened" PLA.
Hope that clears things up a bit. As an aside, if you're not familiar with the different families of nail polishes, and you want to try this, you can tell if the polish you're looking at is "gel" because the bottle will be opaque (keeps UV out).
I would not expect a problem. The greatest problem I would expect would be if the PLA layers were not well bonded, and the coating material could work in between layers. If so, over time the difference in water absorption or thermal expansion could further separate the layers.