I posted a long explanation this morning,....and a power disruption dumped it...D@%n! Anyway:
There are several things involved in rust. They are material, surface condition, environment, chemistry, protection for the preceding.
Material -A file isn't made to be a knife blade. The steel is made to allow many fine grooves to be stamped into it. The resulting structure will have many more pores and micro-grooves in the steel than a piece of HRA 1095. After you grind the surface flat, the ghost of these grooves will still be there. Forging the file will make it even worse. Also, while it was a file, the grooves allowed hygroscopic things, like iron bits and wood dust, to collect and attract moisture. This created tiny pits that penetrate deep into the steel. When you forge or grind the file, you may close up these, or smear the surface over them, but they are still there waiting to become a nice red spot.
Surface - This is where people don't realize that even at 400 grit, the surface is covered with millions of tiny scratches. Often there are deeper scratches and pits that are disguised by the matte finish of 400 grit. Sanding to a smooth and scratch free surface will reduce the places where moisture can collect and rust will form. A smooth surface won't repair a bad piece of steel, but it eliminates as much as it can.
Environment - Merely setting the blade down after forging or grinding will assure that it will be right there when you return....and it will almost guarantee it will be rusted by morning.
Wiping the blade off with a rag will only slightly help, as the pits and scratches are going to hold on to the moisture. Washing the blade with dish soap and drying well in a warm place will prevent the blade rusting as you watch. A light coat of oil is a good shop procedure when working on a blade past the rough shaping stages.
Once the blade is finished, the tiny scratches and pits can still provide a home for mosture...or get filled with things that attract moisture.
Chemistry - We all know that it takes iron and water to form rust. However, there is a lot more involved. Depending on the amount of oxygen available ,the process will be slower or faster. Heat is another factor.Finally, Ph is a huge factor. If you dunk your blades in the same tub you quench in, and rinse the FC off in, and spit/pee/whatever in, chances are good the Ph is lower than 7. The warm blade will suck this acidic water into the pores and set up for rust pits to start digging their way to the center of the blade. Using chemicals on the blade, like FC, bluing chemicals,vinegar, mustard, etc. will also add a lot of acidity to the blade. Rinsing the blade off well will only partly help here.
Preventing the above;
Use a good grade of steel that has not had a rough life in the past. If you are the first person to alter the steel, the chances of getting a better blade will be greater. Avoid any material that was exposed to bending stress, deeply grooved, or badly rusted. There will be micro-spiderwebs going through these steels, as well as millions of micro-pits.
Sand to a very fine surface and then sand downward to a matte finish if you don't want the finer finish. Taking the blade to 1000-2000 grit will greatly reduce rusting and pits. Avoid trying to buff out deep scratches instead of sanding them out . Buffing will close up the pits, but won't get rid of them. They WILL come back to haunt you.
Is your shop humid? Do you set the blade down on the anvil...right over the slack tub? Do you even carefully dry it off? Attending to these, or taking the blade in the house will slow down overnight rusting.
Dump the slack tub often, and toss in a tablespoon of baking soda or washing soda. Put a bit of soda in your dunk can when grinding. This will change the Ph and really slow down rust.
When done with etching or other treatments, neutralize the blade well with ammonia ( Windex) , or a suitable agent for the chemical used. Then wash it well with dish soap. Next, boil it for twenty minutes. This will get all the salts and chemicals out of the blade surface. Dry the hot blade quickly, and the warmth will dry the pores. After five minutes oil with a good grade blade oil, wipe off, and lightly re-oil. Renaissance wax is the king for blade protection as the silicones in it will penetrate the blade and will not evaporate . When storing a blade, clean it well, paying attention to places where moisture can hide. Dry it well for an hour, and then oil/wax it well. Do not store a knife in the sheath, as the chemicals in the leather can rust/pit the blade. It stands to reason to store a knife in a cool dry place. Tackle boxes and car trunks are not two of these places.
One really bad habit is to give a damascus blade "a quick etch" while grinding, to see the pattern, and then just rinsing it off. If you do an etch, fully neutralize the blade before continuing..... or the "quick etch" may return as "slow and unstoppable rust".
Final tip - Use a loupe or magnifier hood to closely examine the blade surface. Use angular light, as it will accentuate the scratches and pits. Keep sanding until they are gone. When people say/ask, " Boy, Bruce Bump's ( or pick any major maker) knives really look great. How does he get the steel to look so good?".......It is because he sands until all scratches and pits are gone. If they won't go away, he probably makes another blade.
