When you solvent weld a PVC ball valve or a union, will the primer or cement damage the plastic ball inside the ball valve or the o-ring in a union if you accidentally applied too much, or the applicator touched the ball as you coat the inside of the ball valve socket?

This happened to a union. Is the o-ring still good?

I’d be more worried about the pvc below the o ring being damaged more than the o ring itself, I would imagine they are both fine
As for a ball valve it’s best practice to glue it in the open position then exercise a couple times… I’ve never had an issue

i always apply primer and cement heavily to the pipe and just a light swipe inside the fitting. just enough in the fitting to get the chemical reaction started. any extra will push off the pipe and remain on the outside.

i makes me angry to watch videos on the internet of installers load the dauber and apply to the fitting first and then to the pipe.

any extra in the fitting has no where to go except to form a blob causing trouble in the future.

Exactly Packy, and which is why you are supposed to use a twisting motion when assembling, to distribute the cement around the joint. The situation in the photo is not ideal, but if it's for the outdoor pond, I would put it together and send it. If it leaks, address it then.

Packy, I try to do the same. In this case though, it is not that easy because some of the piping are under water, and I drained 50% of the pond water for it to be exposed above water for me to make the changes.

The line is 3/4", the fitting socket depth is 3/4", the dauber is 1" long, if I insert 3/4" (full depth), only some of the primer and cement will coat the deeper part of the inside.

Same thing with a ball valve, normally I open the valve so the primer and cement will not get on the side of the ball, but due to some water dripping slowly, I had to shut the valve off, do the connection, then wait a few seconds to exercise the valve because mostly likely, the excess cement get pushed to the ball anyways.

put rubber gloves on and use your little finger as a dauber.

Have the same problem on cpvc, especially on valves not horizontal, you can easily glue the valve shut, so now I use threaded brass valves and cpvc to pipe thread adapters, costs a little more but the valves are much higher quality.