Several thoughts come to mind from this discussion;
1) Elimination oxygen from the blade is the way to stop rust. Water is not the enemy, oxygen is. Make two newly ground blades. Set one on the bench right after grinding and submerge one in a pan of water. The blade exposed to the air will rust much faster. WD-40, oils, waxes ( Renaissance is superb!), bear fat, etc. all are there merely to keep oxygen from getting to the steel. The lower the volatility and the better it sticks to the steel, the better the coating will last and protect. Waxes and very viscous oils are perfect, but hard to get evenly applied. For this reason, most commercial surface protectants have a volatile solvent carrier that allows the wax/oil to get complete coverage of the steel, penetrate all pores and cracks, and then evaporate, leaving behind a thin protective layer of oxygen blocker.
2) Moisture. This isn't the liquid water I was talking about above, it is the moisture in the air. Moisture is water vapor, and it makes the chemical reaction between iron and oxygen faster. The reason a damp blade rust so fast is that the thin surface film of water evaporates, leaving a moist oxygen rich atmosphere around the blade. The thin film of water also provides a conduit for the oxygen in the air to get to the iron in the steel. This is called "wetting".
On smooth surfaces the water vapor dissipates by convection into the air fairly rapidly, but in crevices, dirty spots, or pores/pits, the moisture stays much longer, creating rust lines and pits.
Keeping the shop dry and doing anything to lower the humidity is important. Dump slack tubs when done for the day if practical. Get rid of wet rags and paper towels sitting on the bench ( best to take them out of the shop), and have some moving air in the shop. A small fan running all the time is a good idea if rust is a problem in you shop. Cleaning the blade well with a solvent, wiping it completely dry, and then oiling will keep a blade rust free. Another problem with a wet blade is that as the water evaporates, it leaves behind all dissolved minerals as salts. Just think about your slack tub and dunk pan....what is in that? Do you rinse off blades in them after putting them in the FC tank? Do you spit, pee, dump beer/soda/etc. in them? Does the shop grinding grit settle in them. Is your local water high in minerals or slightly acidic?....well you get my drift - when evaporated, all that will be on the blade surface, waiting to react with the water vapor and oxygen in the air and eat microscopic holes in the steel. Some may not show up as visible immediately, but later on they can come back as rust spots/pits.
3) Sheaths. Tight leather looks great on sexy women, but is not what you want on steel. First, all leather is not the same. Knives are safest from corrosion when carried in sheaths made from vegetable tanned hides. Chrome tanning, brain tanning, oak tanning, urea/salt/bile/etc., and most dying processes use chemicals that leave behind salts which eat steel.
If the sheath is wet formed, it may take a week or more for the moisture to dry out. NEVER keep a knife for more than a few hours in a newly shaped sheath for at least a month.
The best sheaths are not the prettiest. A long term use sheath is wet formed, dried for weeks, and then oiled to become permanently water resistant. It will be tan to brown and rather utilitarian looking, but with periodic re-oiling it can last centuries without damaging a blade.
Knives should not be stored in their sheath. After cleaning and oiling, store in a knife pouch/case, box, drawer, or wrap in anti-rust paper.
4) Cleaning a blade is important. Any small particles of organic material, or metallic salts will attract moisture and make rust spots and pits.
5) Surface preparation is important in rust prevention. The surface needs to be sanded smooth enough to remove deeper scratches and pits in the steel. These places will be breeding ground for rust. One reason old reclaimed steel rusts so easily is it is already filled with deep pits and channels into the steel surface. These can't be removed, and you can't see most of them.... but they will come back to haunt you. "Maybe not today, maybe not tomorrow, but someday,..... and for the rest of the blades life."
Final comments -
Protect the time and effort you spent on a knife. Instruct the new owner on care, cleaning, and storage. Somehow, they always feel it is your fault it rusted on them.
Protect yourself from heartache. After every showing of a knife, even if just between a few buddies, clean and oil the knife. A tiny spot of spittle, a fingerprint, etc. can be a big pit the next time you see that knife. I just spent ten hours of hand sanding restoring a prized bowie knife belonging to Ken Burns from this exact situation. He showed it to some guests and put it safely away in its case. The next time he took it out there were four nice rust pits where someone spittle had landed. (BTW, this was Ken Burns the rocket scientist, not the film guy
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I keep a cloth impregnated with Renaissance wax in a plastic bag at shows. Throughout the day I wipe down the blades as people set them down after looking at them.
