Coming from construction - it is common that hobbyists always lack the preparation and that will always backfire on the project.
So for my rubber roof coating, I pressure wash it first and I do have 3000 psi commercial grade washer.
The original roof had some discoloration, what was 1 of the reasons I wanted to recoat it and some discoloration would not wash.
Than I bought the new-generation $40/gallon silicone coating. I posted it few times already, that IMHO that coating stands several steps above any other coatings and I used coatings in the past, where the silver was the best years ago.
So the new coat sits on my house roof for 2 years and holds well. Only issue on the house was discoloration where the tar was exposed under, but heck, who really cares about flat roof color?
Camper was coated this spring and we did give it good test this summer driving 8000 miles to Alaska, where rains were pretty common.
Coating shrinks, so for rubber roof groves I applied it twice to give it more level finish.