RainX is water based if I remember correctly. So in theory that wouldn't help.
I would look into some sealers or conditioners silicone based. As pwlorraine suggested. Hack maybe car wax would help too.
Rubb it with candle. The wax from candle is rubber/silicone friendly.
BTW vaseline isn't good. LOL That degrades the rubber. That's the reason you can not re-use the condom when you use vaseline too.
LOL
Oh BTW the seals what most manufacturers using is EPDM rubber. Ethylene propilene(propylene? ) diene monomer. Will degrade with petroleum based anything.
Also watch what you use make sure the seal won't fade. Some plastic and rubber I believe EPDM can fade to some chemicals in sealers. Plastic has tendency to fade lot easier then rubber.
But I am not an expert just like to point out things.