All weapons-grade steels (ignoring the new ultra-expensive nitrogen steels) can rust. This is true regardless of whether the blade is made of high carbon steel, medium carbon steel, or fine stainless steel. The best way to deal with corrosion is to prevent it from happening.
That starts with the finish on the blade. The closer that finish is to a true mirror finish, available at great price on normal-sized knives from top-quality custom knife makers, the smoother the surface and fewer footholds for corrosion to sprout. Note that some steels are inherently incapable of taking a great or even a decent mirror finish. On the opposite end of the spectrum is the satin finish. At best it is a series of uniform fine scratches running in the same direction. A true mirror finish is far, far more difficult to achieve than a great satin finish, period. Done right, it adds significant long-term value to the product. The downside of a mirror finish is that it easily shows scratches and is exponentially more difficult to remove rust via repolishing.
Any time a person touches the blade, it will need to be cleaned. The chief problems are salt and acidic skin oil. These are hydroscopic, which means they attract water molecules from the surrounding environment that produce corrosion. The acid itself is also a problem. Acid, including bar soap and liquid hand soap, cause tarnish on weapons-grade steel.
A mere touch can produce a black fingerprint on 440C and D2 stainless steel, and medium and high carbon steel, by the next day. If you leave a pair of swords in your car during the day and it is humid, that can start the rust process. Humidity and temperature extremes can also cause the handle or handle scales to expand and contract. In one case, D2 Butterfly Swords stored in a garage for a season developed dark grey spots that could not be removed. High quality leather sheaths are made with vegetable oil, but lesser goods may use oils that are corrosive and give off corrosive vapor. Plus, particles and moisture can pool between the walls of the sheath or scabbard and the blade. A “D” Guard made from AISI 304 stainless steel is fairly impervious, and cheaper steel D Guards may be very corrosion resistant albeit softer, but I clean my entire sword whenever I need to clean the blade because I don’t like to see smudges.
- The first step of any cleaning is inspection.
- If there are physical debris, try remove that without scratching. I put the blade under a stream of water, tip down so water is less likely to go into the hilt slot, for contactless removal. If that fails, try a wet, soft tooth brush. Martial arts swords should not experience the abuse of a field knife but if you need to wipe, use a soft damp cloth with a non-abrasive, non-acidic detergent and try to avoid scratching!
- Once all debris have been removed, I rinse well with water and non-acidic liquid dishwashing detergent (if you are in doubt, look for a statement on the bottle that it is pH neutral and avoid the word lemon). The blade is, again, tilted tip down to avoid massive run-up into the hilt slot. I also clean the D Guard and handle at this step. Never use abrasives. WD40 may be a fine solvent to remove light rust, and has been written about as safe for cleaning steel, but I personally witnessed it causing some very light tarnish on a high-quality D2 Tool steel. I also saw some darkening on 440C after cleaning it with rubbing alcohol, which is also a theoretically OK cleaning product for steel.
- Dry immediately with a soft cloth.
- Then, let sit to dry in the open since there may be moisture in the hilt slot or so small you cannot discern it by eye. If your blade has a layer coating (e.g., a black finish on top of the steel), moisture will unavoidably be sucked into the area between the coating and the blade so pay attention and do your best during the drying process. Never leave your swords in a pool of water or run them through the dishwasher.
- Next, if your handles are wood and looking tired, or they have contracted a bit, use a soft cloth and wipe them down with mineral oil (be sure the handles are not slippery prior to your next use). If your blade is naked carbon steel, put a light layer of mineral oil or Choji oil (a mixture of mineral oil and a tiny amount of clove oil) on all exposed carbon steel. The exposed tang, if any, on high-end carbon steel knives and Butterfly Swords designed by Modell Design LLC is probably a stainless-steel surround of the carbon steel tang. If the blade is carbon steel covered with a clear coat, you will want to treat at least those areas where the coating has been ground off at a sharp edge or nicked off during use.
High and medium carbon steel Butterfly Swords require more effort than even weapons-grade stainless steel Butterfly Swords. Unless they are coated with mineral or sword oil, or perhaps Renaissance Wax™ (recommended by some knife makers but I have not tried it to preserve high and medium carbon steel blades), the humidity in an ordinary environment is enough to commence the rusting process. Plus, if the swords are oiled, you need to remove that prior to training so your hand does not slip. Treat Damascus swords as carbon steel swords unless you know otherwise.
Use common sense storing your sword. First, use a spot safe from children. Never store any steel blade in or near a leather sheath or case. Store your blades in a dry, non-humid spot at a reasonable temperature. I mostly store my Butterfly Swords on a paper towel in a dry cabinet, out of the sunlight so the handles do not get bleached. Make sure the edge and flat are not under pressure, and pay close attention to where the blades are before you reach for the swords. You may wish to put a desiccant in the storage area to help absorb moisture. Check your weapons for condensation after you transport them, especially if you are going to leave them in a carry case. Likewise, immediately open and inspect when they are first shipped to you! If you plan on storing a weapon in a wood or metal case, remember that the case can seal in moisture. Check your stored weapons regularly.
A lot of Butterfly Swords have brass fittings, as do my doorknobs and Kohler faucets all of which have tarnished. While pure brass cannot technically rust, it gets dull over time and the oxidation of the copper looks a lot like rust to me. The first thing you need to know about a brass D Guard is that it always looks better in the online picture than on the sword you receive. Ignore the opportunity for the manufacturer to re-polish the specimens prior to photography, or the possibility that the manufacturer never intended to polish your swords “to a shiny luster” as advertised, the odds are that your swords sat in a warehouse for a period of time prior to your purchase and the brass dulled during that period. A brass polishing product (one of those fluids you need to test on a sacrificial spot first) can help shiny up a dulled finish, but it is not a magic bullet and I would not want to get any on the blade without knowing the consequences. You may need to use a lot of muscle going back and forth with a rag to polish a brass D Guard, and don’t expect the polishing fluid to remove the not-rust patina. You can get a shinier finish with less work, and take of patina, by polishing the brass with grade #0000 steel wool, and chances are good you will not care about any fine scratch lines. I know of two ways to help slow brass from tarnishing. The first is to gold plate it. The second, as recommended by a locksmith for my doorknobs, is to varnish it. I cannot vouch for varnishing the doorknob since it was less work to just get a new one, but neither method will survive weapon vs. weapon training unscathed. Speaking of D Guards, if you have a steel guard on a Butterfly Sword, and it is not AISI 304 stainless steel, it will require constant thorough maintenance since you touch it every practice.
Once steel corrodes, you should clean it quickly before the spots widen and deepen. Methods of rust removal are beyond the scope of this article since so much can go annoyingly wrong.
I am happy to share my knowledge and experience, and pass on the experience of others, but neither the I nor Modell Design LLC assume any responsibility whatsoever, under any circumstances, to anyone or entity who follows or fails to follow techniques described above (use or abuse at your own risk), including but not limited to discovering that you can cut yourself when cleaning a sword and that it is a bad idea to let it drop on your finger or toe. Be very careful of the pointy tip of a dull-edged sword during cleaning as it may sneak up on you when you are no longer on alert.