There are a few methods to do this. Magnaflux is what I think you are talking about. There is also the acid approach. However I think you only get one shot with that one. Acid kits are available for about $125 with more stuff than you need. Might make sense to contact local law enforcement to see if the have a list of contractor that do that type of work and has the mag kit.
Forensic Ballistics - Recovering Hidden Serial Numbers From Firearms
Greg