We do those all the time as in a hard water area, some are so white you can't even see through them. I use a strong lime-scale remover, put gloves on, add neat l.scale remover to scourer, scour it all over, leave it on for a few minutes, get fairy soapy sponge over the same area and get a glass scraper, gently scrape and scour, rinse and buff, brings them up like new.