It looks like both acrylic and polyurethane should transmit near UV - the sort of UV you need. Getting actual transmission spectra of specific varnish products isn't likely to happen, but I've skimmed a few papers.

Acrylic varnish is commonly used on PLA.

You need to be careful though not to get a UV-curing product (you're unlikely to do this by accident), but also not to get one designed to slow the effects of UV of the item, as some wood varnishes are supposed to do.

It looks like both acrylic and polyurethane should transmit near UV - the sort of UV you need. Getting actual transmission spectra of specific varnish products isn't likely to happen, but I've skimmed a few papers.

Acrylic varnish is commonly used on PLA.

You need to be careful though not to get a UV-curing product (you're unlikely to do this by accident), but also not to get one designed to slow the effects of UV of the item, as some wood varnishes are supposed to do.this is a selling point, so you'll need to look for varnishes that say nothing about UV - and probably run a test to be certain.