i tell them its 3 cleans till the frames start shining, its true too/
you can scrub as hard as you want,they rarely shine on the first visit. but wiping them down on 3 seperate visits they suddenly come good. the 3= clean= story ties them in a bit too, by the 3rd frame scrub theres a bit of loyalty created ,of sorts
i just use my usual scrubber that i use on the glass ,on the frames and wipe with scrimcloth after
id never use special chemicals ,just use washing up bucket water, if you use anything harsh there could be a comeback later