Should you let PVC primer (purple) dry before applying cement or is it okay to apply cement over wet primer?
No - PVC Primer does not need to 'dry' before use - in fact, it needs to be wet to do it's job and not impede the cement - remember, plumbers in a hurry use this daily and aren't sitting around waiting for the primer to dry before grabbing the cement.
I make sure everything fits first, then grab the primer and swab it on, set it down and grab the cement and swab it on. If you wait - it's likely no more the 10 seconds. If you've waited a few minutes because you've gotten distracted, made a mistake or something and it's fully dry, just reprime to be sure the plastic is accessible and softened for the cement. If you don't- it can prevent the cement from bonding as well to the plastic.
The cure-times listed on the can are for after the cement cures- typically 2 sets of times will be listed - one for handling and one for pressure testing.
This answer is correct, not the the previous one. See Oatey's instructions on the can or at oatey.com/products/plastic-pipe-cements-and-primers/… - it reads "Additionally, after application of the primer, the solvent cement should be applied immediately before the primer dries."– BadZenJul 9, 2016 at 22:11
Actually the brand I use (weld on purple) says you must wait 20-30 seconds at 70-100f, 60-90 seconds at 40-69f pipes larger than 1 “ a second application is required. This comes from the Weld-on label if you don’t wait long enough the plastic will not be softened (primers job)– Ed BealSep 23, 2019 at 16:25
The primer needs time to do its work. In the summer time I wait for it to be dry. In systems that are using higher pressure or cold weather I really do wait several minutes. The instructions do provide times for size and temps but my can is not readable. I have only had 4” at 90 psi fail but after that I did start waiting longer after reading all the instructions. I think there is some safety built into the suggested times.