actually im "insta rusting" a case right now, there are two components to the insta rust the metallic acrylic paint, basically tiny fragments of iron in the dark grey acrylic paint and then there is the rust liquid which is then brushed on after you coat the metal. I tried at first just using the insta rust liquid and it started to build a little rust but more than anything just oxidized it with no rust. After applying the insta rust on top of a layer of the metallic paint bam 2 hours and I had rust forming. Now the only problem im having is that on the top where i left the original paint its forming extremley good, however on the bare metal even after two coast of the metallic paint its still barely forming. My suggestion would be to coat the entire case with a layer of primer, probably just a quick spray of krylon would work fine then paint on the metallic paint then let dry and your would have some good rust formation.
As for it flaking off, well its very very minute rust almost like just a layer of powder rust forming not deep into the metal like it would naturally so yes it will come off just by rubbing your finger across it but not all of. What I would do and plan on doing is after the rust is formed to my liking im going to spray it all down with a sealer and clear coat to get everything sealed in..
Ill post some pics sometime tomorrow to show you how its progressing..