After that exchange, I dabbed some Oatey PVC Purple Primer/Cleaner on two PLA slabs:
The active ingredient involved in PLA bonding is tetrahydrofuran, which makes up anywhere from 10 to 40% of the primer (the MSDS gives a broad range). The primer immediately marred the PLA surface, which is exactly what you want in a solvent adhesive.
After an overnight clamping, I couldn’t pull or peel that joint apart: the two slabs had become one. That’s unlike the paint stripper test that didn’t bond well at all. Good enough for me.
Obviously, you’d prefer Clear Primer for natural PLA, but Purple Primer is what I had on hand.
Given that this stuff has no solid content, I think it’s more suitable as a PLA adhesive that the thicker PVC Cement. However, clear cement would be less likely to run along the thread seams and ruin the surface finish outside the joint than water-thin primer.
Tradeoffs, tradeoffs… but now I can build things from PLA subassemblies!
The Smell of Molten Projects in the Morning 