Tires losing air pressure

I have an '83 GPz550. I put new Avon Roadriders on it about 3 years ago. New OEM valve stems as well. The rear holds air pretty well - maybe loses a few pounds per week especially in cold weather.
The front loses substantially more air - from about 35 psi to 15 or so psi in one week. Immersing in tub of water, I can see bubbles out of several areas where bead meets the rim.
I've had it remounted where the tech has cleaned off the crud and oxidation off the inside part of the rim.
The rim is not bent, in great shape overall.
But it still is losing about 20 pounds per week.
Looking for suggestions on how to resolve this - further smooth/sand inner side of the rim? Switch tire brand to one with a better bead? New rim? Remount using liquid gasket?
Thanks!

I don’t know the name of the material but every tire I’ve seen mounted on aluminum rims get a liquid black sealant on the inside of the rim bead before tire mounting. Of course after rim is cleaned. I see them use a scotch bright type disc on air grinder

Put a tube in it ?

I have never had a set of Roadriders, but I have worn out a fairly large stack of Avon radials, but have never had one loose air like your rear tire, and certainly not like that front one.
Like you, I would first suspect the sealing surface of the wheel. A deep scratch or two is all it takes to create a problem. If the surfaces are smooth, clean and flat, you may have a faulty tire. I have only one warranty experience with Avon and it was a good one.
I have never had any black goo applied to any wheel, ever.

Thanks all.