All 2 part epoxy products will go chalky - hence why a nice new gloss black will go 'grey'. An ultra high pressure wash (so a washer more powerful than conventional pressure washers) will likely restore the black finish, but not the gloss effect. I use interzone 954. It's not cheap, but it was developed for use on oil rigs and offshore platforms, so it's pretty durable and is supposed to be good for 15 years (although I can't vouch for this claim). As per previous posts, the key to applying any epoxy is to follow to the letter the manufacturers preparation requirements, otherwise it simply will not bond.