Bubbles,
Thought I'd see you in Vegas...
Made my own (I do all my own signage with the vinyl cutter I purchased a few years ago...save a bundle and do it my way). Having said that, all dog owners around here are familiar with oatmeal treatment shampoo and my stink-buster shampoo is self explanatory (IMHO). So we haven't had many questions about it. But if you listen to your customers and watch how they use it, you'll then figure out what's the best signage. Also, as a rule I find people don't read signs.
One thing you may consider, is to make a video (like the one on my webpage), get a cheap LCD screen and small speakers and mount it in the petwash somewhere safe from splash (or in a housing). Then run the video non-stop for a while until people get a sense of it. I find that it will be way more effective.
Overall, if dog washing is new to your area and/or your customer, you're going to get a lot of people coming to check things out. This is a perfect time to take 2 minutes and explain how it works, the different functions, etc. It will take time to build your customer base...I've been at it for over 5 years and its still building my customer base.
How about some pictures!
Big