From: cindy 

I am sure that everyone who has indoor cats has this problem
The lovely crust/ film that forms on the litterbox that you scrub 
and scrub at. I know some people resort to toilet bowl cleaner 
and steel wool to get it off. I accidently found some stuff that 
works great. The once a day shower stuff that you spray after 
the shower on the walls to remove soap scum.

Wet the litter box and spray the box and come back later. Rinse 
and you will see the crust is lifting. I found this by accident after 
I put the empty box in the bathtub to clean and decided to 
spray the shower and some got in the litter box and made a 
phone call and when i came back the crust was lifting.

Now this is a newish box but I bet that on an older box over time 
that this would make the boxes shine.

Cindy