If the rubber surfaces are powdering to begin with I would imagine using a spray on enamel or lacquer coating would eventually result in it flaking off in a particularly unattractive and likely distinctly annoying manner...
Bar Keeper's Friend. It's cheap and will clean and polish the entire keg, making it look new.
__________________ "What has been will be again, what has been done will be done again, there is nothing new under the sun."
"... in the US, people need a style first and then a beer. I am still convinced that it's reverse; you enjoy a beer, not a style. An artist creates, a publisher copies. Brewers should be artists, but we are constantly bogged down in trying to diminish the art into a style." - Peter Bouckaert