I fit a 10-8 sight on my M&P recently and I'm having trouble bluing the exposed metal on the front edge of the dovetail. 10-8 said to try Brownells cold blue, but I picked up some Birchwood Casey since it was available locally. Didn't work at all. I'd hate to have to send it somewhere to get it redone with the black salt bath nitrocarburizing finish. Any ideas?