You might try some Brownells dicropan im bluing applied as a cold rust bluing, no boiling water. I've used it for several years on guns and gun parts with good success. Polish the way you want it, degrease it with lac thin or acetone, apply a coat of im, let set for a day, go over it with steel wool, then give it another coat. If you don't get grease on it you don't need to degrease it again. I use a toothbrush to apply a thin coat of bluing. Usually takes about 4 to 6 coats on softer steel like barrels, maybe a couple more coats on harder steel. After all the coats wait a couple of days then oil. It doesn't take long to apply or very much to blue the part. It seems to become more durable after a few weeks. touchups can be done by degreasing and applying a couple of coats. I've used this to touch up hot caustic bluing by steel wooling where it needs it, then applying a couple of coats of im. Cannot tell the difference between old and new bluing.