I have a mix of solvent welded (glued) fittings and threaded fitting pvc piping for my above ground pool fittings. The solvent welded fitting are always leak free, while the threasded fittings seem to leak often. Can I solvent weld the threaded fittings?

If the threaded pipes are the same PVC material as the solvent welded pipes, then the glue will have the same affect. As you know the glue sets up fast so when you're rotating the pipe orientation might be compromised.

Wrap thread with Teflon tape. For 3/4 inch fitting wrap 4 times. For each 1/4 inch larger wrap 2 additional times. Be sure to wrap whole thread. If tape does not cover all of thread with one wrap, count total wrap as one time around.

Chuck