A friend suggested to me today that it might be unwise to put Deri-sal on Penny's paws, because she might lick it off. So now I'm not sure if I will use it again. If I do, I'll put a bandage around her paw so she can't lick the salve.
Next time I'm at the vet clinic, I'll ask for his advice, and I'll write a post about what I find out.
I thought this thread about horse injuries was very interesting. I was once told that liniments and salves for horses are suitable for dogs, because their skin pH is similar. However, on reading this page I find they are somewhat different.
PetMD has a good summary of what we need to know about canine skin pH, in relation to shampoos, and I guess the same would apply to skin medications.