I have done a lot of glass work inside steel tanks and concrete chemical containment sumps where it was not physically possible to turn the work upside down and let gravity work in your favor. It is possible, but there are no shortcuts and it isn't pretty. For one thing, the resins used were either special chemical-resistant novolac epoxies or platelet-filled vinyl esters, and both are high-viscosity materials. I could give you a laundry list of tricks, but the best solution by far is to let gravity work for you.