Black spray paint works great and easy too. Place the tank outside on its side, tape off the other panes and go nuts. You really can't over do it. When I painted mine I placed a flash light inside shining against the inside pane your painting. This helps to make sure you get even coverage. Paint until you can't see the light anymore.
I took a different route when weather proofing my stand. I painted the who thing with liquid rubber and then wrapped everything in vinyl and sealed the seams with silicone. Another option, if you want to get crazy, is you could take the stand to one of those places that does rhino lining for truck beds. I considered this option when I built my stand but if I remember correctly it was a couple hundred bucks but there would be no doubt that your stand would be 100% water tight.