AT LAST! A topic on the
RPF where I can help due to my experience!!!
I've tried it with wax and it worked pretty well. The best thing is to melt candles in a pot and then apply the wax with a brush (a throw-away brush, it won't be very useable afterwards).
After the way cooled down, you can scratch it off just as you said. But be careful, the more filigree the design you scratch off is, the more likely the wax is to break. AND you should pay attention that you apply the wax very thoroughly. If you have any air-intrapments under the waylayer that touch the scratched off areas, the acid can flow underneath the wax leaving some nasty etched areas you don't want.
My advice: try it with some scrap-metal first until you get an idea of how much wax should be applied and so on. Then you can also use fingernail-polish instead of wax, just to see which one works better.
Oh, and another thing, how do you want to apply the acid?
I asked my chemistry teacher before I tried some etching, he said it would be best to dip the metal into the acid, then dip it into water, and repeat if depending on how deep you want the etched areas to be. It you want to lay the metal down and apply the acid (with a brush, for example) and just wait (i.e. not dipping it into the water), it could happen that the etching becomes uneven.
Hope that could help.