skin problems

Hi I am new here and have a couple of questions.
We have two big dogs. One is a rotti shepard x and the other is a bull mastiff great dane x. The rottix has had skin problems for awhile. We have treated him for fleas and ticks. He seems to bite at himself, especially on his rearend and tail. He can make such a mess of his skin. We've had a hood on him for a couple of months now. We also have tried antibiotic cream and tea tree spray. It doesn't seem to help him and now our other dog is getting the 'hot spots' too. He is short hair and the other dog is long hair. We don't think it is flea related anymore. They stay at home in the yard or inside. They don't socialize with other dogs. What is it? I've heard about Topagen spray, does is work? and is it worth a try? Also the bullmastiffx has lost the hair where the hot spot is and it is growing bigger, about the size of a softball.
thanx and I'd appreciate any info on this matter
have a good day all

Topagen does work well, I am putting it on my Golden with mange and it really does help and cleans up the skin it is a antibiotic. Maybe you should have the dog anal glads checked by your Vet. Poor thing wearing a hood that can't be nice.
Good Luck

Yes!! I used to have a dog that chewed alot and went bald at the back end from chewing. Tried all sorts of flea products believing it was from fleas. Then a vet told me to change the diet. Said some animals have reaction to some of the food. Put her onto lamb and rice food and never had a problem since then. Maybe you will want to give that a try... Good Luck!! :)