I received the white decal powder the other day, and today I finally got around to try it. It's really easy to use, you pour it on top of the printed decal while the ink is still wet, then immediately remove the excess powder, and heat with a heat gun. The heat gun will cause the powder to melt, I assume it's tiny tiny plastic pellets, and become a thin plastic 'skin'.
Unfortunately, I'm not all that impressed with the result. First, the edge of the letters are not very sharp, so I doubt it can be used for anything much smaller than what I printed, The "C" is almost 1" tall, second, the plastic seems to be floating instead of being attached (or in my case I was looking for painted look) to the wall. Third, since the plastic is much thicker than if just printed, I don't think it can be used on any textured surface. In this case I had hoped to have some of the wood texture showing, but as you can see, no success.
Of course this was only the first test, and I tested it on a piece of test wall I had build to test some stain color, and wasn't too careful when placing the decal, so maybe with a little more patience and a little more care it can actually be used as white decals, just not as a 'replacement' for a white painter sign on a wooden wall.
Jesper