I have several stainless bowls that I use for dog water, keep them full so if I am gone (or die) dog may be hungry but not thirsty until someone shows up or I get back. Anyway, there is a scum on bows that is hard, doesn't come off. What should I use to clean off that won't leave a dangerous residue for dog? CLR? It must be dog safe. Large plastic buckets have plastic residues that can be dangerous. Thoughts?