If the 'scum' is scale from hard water, filling a spray bottle with a strong solution of vinegar would be a good idea. Give the stuff a good spray-down while it is dry.

Fantastik could work also!

I did cleaning for a while several years ago, and did a quick clean for a fellow whose sister was coming to visit. This was in Vancouver.

His bathtub was pink. Eventually. But the WHOLE inside and some of the edges were GREEN!!!!

I think I used comet - maybe fantastik - and a ton of elbow grease!

Another excellent cleaner is TSP. A little goes a long way, and it is incredible. Look it up on the Net and see if it is indicated for scum (the shower kind, not the customer DID I SAY THAT???? )