Oh boy, I hear you on that one
Our rotti Slater (who went to Rainbow Bridge earlier this year) used to get hot spots every spring. Poor fella, it's not fun. Seeing as it happened each year (like clockwork), our vet was nice enough to tell us to not bother bringing him in anymore after the 3rd time. She allowed us to stop by and pick up a scrub for him as soon as he got it each time. Always worked, would never spread when using this scrub...
Kind of smells mediciney/soapy, is called "Hibitane Scrub", is pink and works wonders. Just have to scrub (kind of hard) on the hot spot, rinse, then dry. Feels good too (his eyes would roll back when we did it).
Dont' know if you can pick this up (but) it sure works (and isn't expensive)...
Hope that helps - good luck