My old multiport valve was hard plumbed to my filter. When I replaced my laterals I thought I'd convert to a union style mount for easy future disassembly. Someone had goobered one of the threaded connections to the valve with silicone, so in hindsight I should have suspected what I was soon to discover. After re-plumbing everything I had an every 5 second drip from one of the valve connections despite multiple attempts at tightening it. Took it all apart and sure enough, found a hairline crack through the threads and into the valve body. See the photos.
Read up on the details of solvent welding and learned that the purple primer is composed mainly of solvents which are intended to initiate softening and swelling of PVC. Knowing how thin that stuff can be when fresh, I reasoned it might wick into the crack, so I lavishly wiped primer on the crack and all through the threaded area. I then dabbed regular clear solvent weld on the external crack only and left the threads in primer. I reassembled with a schedule 80 PVC close nipple that I also wiped with primer and a week later I still haven't seen any leak.
I probably just got totally lucky and it's hard to say if this will last, but if it does last, it saved me much unnecessary expense.