I have restored 2 Mueller brand hydrant in the last 2 years. The neighbor up the road has an industrial sandblasted & charged me $40 to blast it. I brushed on 2 coats of new holland yellow on the bottom & the same with Massey furgusen red for the top & caps. Taped it up & sprayed the 3rd coat for that glossy finish.
I recent did another one that was finished all red. I had sandblasted it myself since we have 2- 2-stage compressors in out shop. Might have taken 200 lbs of silica sand to do the job, but very time consuming since the compressors couldn't keep up the sand blaster. A chemical based solution u can try Muscle Magic from o'Rieley's. We always got the 5-gallon jug sine we use it to degrease tractors, it also helps "cut" through the paint. However, we spray Easy Off Oven Cleaner (has to be the brand named) on the project, let it sit for 5 minutes & scrub it with a scotch brute pad & then wash it off with hot water. Just some things we use when it coming to cleaning cast iron.