You do not want anything on the rubber gasket.
You want a clean rubber seal between the tub and the shoe.
Silicone goo or putty create a small gap that allows water to seep through.
It the tub has "rough fiberglass" at the drain outlet, then it is a "defective" tub.
The surface under, and around, the drain opening MUST be flat, smooth, and parallel to the tub's surface.
Well and good. I agree.
But here's my situation....
DEFECTIVE GLASS TUB; DRAIN FLANGE NOT FACTORY SMOOTH.
Little Old Lady, fixed income; I need to patch this situation up for her for under $150. Call it "defective" and get a new tub enclosure is not an option.
Very Minimal Access. I have a 16x40 opening from floor up in the tiny towel closet behind the tub; I have trouble getting into the closet. The subfloor around the drain is cut out really close to my drain pipes; I agonize at the thought of attempting to reciprocate in there.
No way I'm getting a 4" grinder in there to to flatten the flange without causing extensive damage to custom plaster work or rebuilding the closet. I have no intention of involving myself in this and she doesn't have the budget. No access from below without removing finished work: custom cabinets, crown moulding, etc. etc. Custom finished fancy plaster work; I don't want to have to talk her into a hole in the ceiling that I or another contractor will need to repair.
Fiberglass drain flange is factory rough; aka "defective". Previous plumber used "plumbers putty" between the bottom gasket and the unfinished rough glass on the bottom side of the tub where there should be no putty or silicone... just a gasket as you've stated. As expected, plumber's putty has since degraded... major drain leak; causing minor damage to ceilings below, which is still reconcilable without tear down.
Currently a screw together PVC type drain assembly w/ lever. I could be talked into brass replacement.
Thoughts on epoxy or glass resin and clamping it shut w/ rubber gasket while wet/hot?
3M 5200 Marine Sealant?
Vulkem 116 Sealant?
DAP Butyl-Flex 27062?
X-Pando?
Hercules 15703 Blue Block?
Phenoseal?
Open to suggestions. "Applied Product" preferred. I can get in there with a sanding block and probably knock the high spots but it will likely never be a factory "flat, smooth, and parallel".
I'm leaning "hand sand it", "brass trap", "double rubber gasket", and plenty of "Hercules 15703".
Thoughts?