I was wondering if any more experienced plumbers can assist me with this. I recently witnessed a company that was applying teflon tape and pipe dope to a brass union on an antiscald valve for a bath tub. In my experience (I have only been plumbing for less than 5 years) with brass unions such as those on a pressure reducing valve, the rubber (or fibre) gasket was sufficient for sealing it. Am I correct in thinking that the small rubber gasket with built in strainer on these unions is sufficient for the seal, and that the tape and dope is not necessary? Or am I wrong. As I said, I have only been plumbing for a few years and welcome any more experienced feedback. Also, I am plumbing in Canada so not sure if any Canadian plumbers can correct me if the code here says otherwise. I don't remember reading it but I haven't received all the updates since I got my ticket.
The only time you need to seal the pipe joint with pipe dope or tape is on a tapered pipe thread. Any other connection is sealed with some sort of gasket (like a hose on a hose bib). It doesn't hurt to put a little pipe dope on the mating surface of a union, but tape will often create a leak.
Putting tape or compound on a gasketed joint does absolutely NOTHING to help it seal. Compound on a ground joint union help seal any small imperfections which could cause leaks.