No real need to mask off anything if doing an Express Blue job,,just avoid putting any soln over those areas.
If slow rust bluing, I use common shellac to mask off areas that I don't want to be rusted and blued. I also use shellac inside the bores on bbls that are being damascus finished as those go thru a dunking in ferric chloride soln and will etch the bores each time if not protected.
The shellac holds up just fine thru repeated boiling cycles. I sometimes re-coat the bores in a damascus finish job after 6 or so cycles just to be on the safe side. Damascus finishing can take 12 and more cycles. Rust blue only 4 to complete. I boil after each rusting instead of building a rust brown up over several coatings and then boiling once.
Just the way I've always done it.
You can stabilize some active rust by boiling a part like a slide. It'll turn the red rust to blu/black rust just like the regular rust bluing process.
However you may end up with a blotchy off color look to the areas as the rust has not formed uniformly as in the controlled rust bluing process. Plus the finish of the rest of the slide is glazed and pores filled with old dried oil and what ever else from years of handling and use
Often it can be made to look quite uniform by first gently hand polishing the entire part just enough to break through that surface and expose some fresh metal. No need to even remove the bluing that is there. You will scratch it but only to the extent of the grit size of abrasive you are using and that is the goal.
Match that to what the original polish is underneath the blue and you won't disturb the original look of the piece when done.
When it's all cleaned up, degrease it and go ahead with your rust bluing on the part. Apply a coating to the entire part. Either an Express Blue soln coating or let it rust in a Slow Rust cycle. Don't let it rust too heavily in the latter, It doesn't need that heavy red coating to produce a layer of blue when boiled.
When it's ready, boil the part and check the color. It may be all it needs. Perhaps one more will do it. Covering old blue both hot salts and rust types can be done quite well with rust blues if you pay attention to how the solns and the rustings are responding.
Slow rust blue solns rusting times vary with humidity and temp of course.
But even in winter, I get a rusting cycle in about 20 to 24hrs using LaurelMtn soln. It's 64F in here all winter and humidity is ?. I just hang the parts and keep watch on them till they're ready. Sometimes that first coat needs a re-coat before they take off and start rusting. Again, no need to build up a heavy orange/red coating of rust on the parts to get you a coating of blue color. Just a light hint of brown on the part, sometimes I have to see it in better light than the simple overhead light in the room. Usually I very lightly drag my fingers down the hanging part(s) and feel the coating and know it's ready.
If I want to really speed up the rusting process, I hang or prop the parts up in a shower room. Turn the shower on for a couple minutes to steam up the room. Then turn the shower off and leave the door closed.
Don't let the water get on the parts, you just want them subjected to the humidity and temp. Not humid enough to cause water droplets to form on them either as they will spot the finish and ruin it.
They'll be rusted nicely in about an hour to 2 hours.
You can do a slow rust complete job in a (long) day that way.
Most chemical solutions don't have mercury in them anymore like the older ones did. (Herters Belgian Blue sold by Brownells as a new product may still have it, the last time I tried that,, it did).
I simply stay away from any that do,,but another thing to watch for is if you do use any of them (usually Express Solns), the mercury will plate out onto any non-ferris metals on the gun when applyed. So anything with gold inlays, brass & copper(sight beads & bars), ect will come up with a terribly tough adhering coating of silver mercury on them.
It does not just rub off,,It's there to stay.
We used to have to burn it off w/a torch to remove it (wonder where the shakes come from?)
Just stay away from anything w/mercury. Plenty of perfectly good rusting soln's that don't have any in them.
Yes they worked great (Birchwood Casey Plum Brown old formula was another one w/ Mercury Bichloride), but they aren't worth messing with.
