Is this the new wave? Some awesome footage that I found on the SMART crew page. Now that this stuff is no longer really a "secret" anymore, we can feel free to share it with you. Before going any further, please read the safety precautions below CAREFULLY!

WARNING: DO NOT ATTEMPT TO USE THIS CHEMICAL MIXTURE INDOORS (unless area is properly ventilated) THE FUMES CREATED WHEN MIXED WITH METAL ARE TOXIC! When handling the mixture, rubber gloves, safety goggles and a respirator are highly recommended!

METAL ETCH RECIPE:
You will need the following
- 70% solution of Chloric Triphosphorus (in liquid form)
- Plastic container and a metal pot that the plastic container could fit in *optional
- A standard 12 V, 40 Ah Lead-acid car battery *optional
- Rubber gloves, safety goggles, respirator, apron (recommended)
- A plastic mop with tons of extra tips
- Some heavy duty garbage bags (contractor types work fine)
- Roll of paper towels

Lay out some of the garbage bags in your work area and have a bunch of paper towels ready. In case of an accident, DO NOT dump this mixture down your drain as it will RUIN your metal pipes. Just soak it all up w/ paper towel and dispose of in garbage bag. You can actually use that 70% (you can even use a higher %, but it really makes no difference) solution of Chloric Triphosphorus in your mop directly but it won't give you that rust effect on the metal that forms the next day.

To get that rust effect, you need to get that battery acid involved. I won't go into detail about how to get the acid out of the car battery (I believe it can be used/un-used although I'm not 100% sure) but you can figure that out yourself. You only need about a 90% (Chloric Triphosphorus) / 10% (Battery Acid) mixture. Adding too much battery acid will actually weaken the effect. Pour the mixture into the plastic container and then submerge the plastic container into a pot of boiling water to heat it up. DO NOT pour the mixture directly into the metal pot for obvious reasons. Let it cool and dump the mixture into a mop. This mixture works on all types of metals w/ different effects on different types (of metal.)
Disclaimer: DO NOT use this for vandalism of any kind. This is merely an instructional post to show the wonderful possibilities of art projects you can accomplish with this new medium! Again, DO NOT USE THIS FOR VANDALISM PURPOSES.
http://blog.smart-crew.com/ for more info.