The two places you’ve identified are absolutely notorious for leaking, that’s probably why you found sealer on them.
If everything is perfect, you probably don’t need to apply sealer ( even so, I still do , because I absolutely hate leaky petcocks!)
Here are some tips for leak proof petcocks. First a disclaimer, I have aftermarket petcocks on both of my bikes, but the theories still apply.
1. That black gasket for the back plate is tissue thin and will not seal if there are surface imperfections. They will seal if the surfaces are perfect, but I like to make my own gaskets from a good gasket paper.
So, this idea came from Gary, Wet sand all the flat surfaces with 400 grit to make them absolutely smooth and level. Gary also recommends always replacing the internal seal, that round rubber part with the four holes, with OEM Yamaha parts.
you can see the difference from original on the left to sanded on the right
The mounting screw sealing washers are also problematic, sometimes they seal, sometimes they don’t, and I don’t recommend ever reusing them. I go to either Ace Hardware or True Value and I buy these small, rock hard grey fiber washers that are just a hair smaller than 6mm. They are indestructible and reusable. You can run a 6mm tap through them and thread them right onto your mounting screws.
Here’s a good tip from Jim, concerning leaks around the o-ring. Take a wooden popsickle stick and grind it to a rounded point and use it to spread black RTV silicone , to create a better seal for that o-ring. Be sure to let it dry for 24 hours. I’ve done this , it’s easy and works great!
And lastly, I usually use Hylomar Blue to coat the back gasket and the sealing washers, with good results. Just be sure to let the Hylomar dry before assembly.
These are all just things that have helped me in the past, none of them are absolutely necessary, if all of your sealing surfaces and gaskets are perfect, you shouldn’t need to do any of this. But if you continue to have trouble, some of these ideas might help. Trust me, a lot of us have struggled with leaky petcocks. I hope this helps. Good luck, Bob