we have found over the years that NONE of the "cold blues" will work all the time , on any given gun, sometimes it does and some, not...thing is it will clean off relatively easy if it does coat...the trouble is they are 'temporary' and in some cases will smudge, and not blend in with the surrounding area, in fact make MORE of a mess.....you need to buff and polish, remove the old bluing and properly "hot dip" or even the old fashioned 'rust blue", which is time consuming and very costly. I have tried Vans and for us ,it did NOT work, and we have tried about all of them on the market, with little success...be careful not to screw up what little bit of 'blue' is left on the gun....Smiths & Colts are the hardest to try and 'blend' (touch up) some of the other gun makes, 44-40 ( Numrichs' brand) tends to do the job....bottom line try different ones and try different ways of applying, warming the metal surface tends to help, but again, doesn't "last".
The thing is, many of these 'cold blues' will work on 'bare metal' pieces of carbon steel and even nails, but once a firearm is "blued" ,you are dealing with a different situation............................. Good luck......................