Technically, any filament can be smoothed if you’re willing to use sufficiently noxious chemicals. Ethyl acetate is apparently the least noxious smoother for PLA—it’s used in perfumes and can be purchased off Amazon, Ebay, and other large marketplaces. Not something you’d have lying around, but I don’t think it’s terribly toxic, either (never tried it myself). Still, check some MSDSs or the like before you shoot off and buy some.