Dave,
You are not tanning your skins, using the above, you are simply removing the fluids. Eventually those skins will deteriorate. Part of the tanning process is to remove those same fluids but also to put a tanning agent in to replace. The funny part of taxidermy is we "remove everything and then put it all back in". Tanning is really a neat process dealing with acids, alkalines (sp?), chemical reactions, biology, etc. Sounds difficult but really isn't. I would recommend having your taxidermist order you a tanning kit, available through most major suppliers, I prefer the one from McKenzie's. It is really easy to follow and understand. Very little checking ph and little thought accept following directions. You will find that with an actual tanning agent, you will end up with a much more durable and stable skin. Good luck.