How we did ours when we had them....
Hot water...very hot so wear rubber dishwashing gloves. Put in a small bucket with a capful of clorox clean up and get some of those sponges with the scrubbie side and start scrubbing. The very hot water gets the old grime off. They should come clean fairly easily unless they are very scratched up then they might not come out as well but will still be clean at least. Go back with just clean water and do a light rinse so they don't have soap residue.
"A little less chit-chat a little more pitter-pat"
Twice a year I pressure wash to get the dust off and loosen up the grud then I use the stuff you use on Aluminum horse trailers, sorry I don't know what it is called. I get it by the gallon, dilute a small amount in a bucket of water, wipe down with sponge or soft brush (wear gloves). Then I pressure wash again to rinse it, they look brand new when finished.