Woodworking Tip of the Day: Keeping Rust Off Your Tools in a Salt-Air Shop

Salt air's good for the soul and murder on a plane iron….. Work a few hundred yards from Marblehead Harbor and you learn it fast. Leave a block plane on the bench Friday, open the garage door on a mild, damp Saturday morning, and there's an orange freckle on the sole and a fingerprint etched into the iron. I'm building a Chamberlain Marblehead gunning dory, and the bronze fastenings don't care about the weather. The plane irons, chisels, saws and cast-iron tops do. So today: why tools rust, what the humidity numbers really are when there's salt in the air, and which coatings actually held up when Fine Woodworking tortured twenty of them.

Why it matters: Rust is iron, oxygen and water getting together. Fine Woodworking's Thomas McKenna lays out the two things that speed it up in a shop like mine: salt, whether it's from sea air or the moisture off your fingertips, and temperature swings that cause condensation. In an unheated garage, when a cold night turns into a warm day, condensation settles on the saw table, the jointer and any hand tool left on the bench. Once it starts, it spreads like a rash. A little surface rust on a chisel back or plane sole is an hour of flattening you didn't need to do. Pitting is worse, and if it reaches the edge, you're grinding the pits out.

The numbers that matter

• Clean steel: CoatingsPro (an AMPP corrosion-industry magazine) puts the critical relative humidity for steel at around 60%. That's the level below which water won't form on a clean metal surface. At 75 to 80% the corrosion rate jumps sharply.
• Salty steel is a different animal. Sea salt pulls water out of the air. Plain salt (sodium chloride) turns to liquid brine at about 76% humidity, but a 2014 study in the Journal of The Electrochemical Society found steel dusted with salt crystals could start rusting at as low as 33%, and from 53% up it rusted nearly as fast as steel sitting under wet brine. Translation for the coast: a shop that feels dry can still rust a tool that has salt on it. Wiping the salt off matters as much as the humidity.
• Dew point: The National Weather Service defines it as the temperature the air has to cool to before it hits 100% humidity. Anything colder than that collects water. Work it out (my arithmetic, using the standard Magnus formula): 60°F shop air at 70% humidity has a dew point of about 50°F. A plane iron that sat at 45°F overnight will sweat the minute that air hits it. Woodworker's Journal's Tim Inman calls it the sweaty iced-tea glass, and points out it's the changing temperature that does the damage, not the cold itself.
• A dry shop: Woodworker's Journal's Chris Marshall runs a dehumidifier year-round in a closed shop, and his humidity hovers around 40 to 50% in the mild months. That's the range I aim for.

What actually works: the Fine Woodworking test

In 2012 Fine Woodworking put 20 rust preventers through ten days of abuse. They coated a cast-iron table saw top, spritzed it with water and left it outside under an overhang. Then they coated A2 tool steel samples and froze and thawed them every day so condensation built up. The saw top showed rust after one night, and the tool steel within 48 hours.

• Top seven: CRC Industrial 3-36, LPS 3, Moovit, Rust Block, WD-40, WD-40 Long-Term Corrosion Inhibitor and 3M Rust Fighter 1.
• Their picks: CRC 3-36 was best overall. It has a petroleum odor, so if you'd rather keep petroleum off your hand tools, they recommended Moovit. LPS 3 and 3M Rust Fighter 1 performed perfectly but left a sticky feel, and LPS 3 was the only product that kept yellow glue from sticking.
• Finishes: None of the products interfered with an oil-based or water-based topcoat, even when they put them right on the wood.
• The surprise: Paste wax is a fine lubricant on plane soles and machine tables, but it didn't offer much rust protection. Camellia and jojoba oil were marginal, and Boeshield T-9 didn't make the top seven.

Now, to be fair to the oils: Lie-Nielsen tells its customers to keep iron tools lightly oiled or waxed and recommends jojoba, and they say it won't interfere with finishes or gluing. And a ship's carpenter writing in Fine Woodworking took camellia oil and several other rust preventers along for nine months on a tall ship, with salt spray and sweaty hands working on his tools, and camellia oil beat them all. Here's how I square it: a thin wipe of natural oil is fine on a tool you pick up every day. A tool that's going to sit in a damp garage for a month gets something that passed the test.

Materials

• A rust preventer that passed the test: CRC Industrial 3-36 for machine tops and idle tools, Moovit for hand tools
• Jojoba or camellia oil for the daily wipe-down, if you like a natural oil
• Paste wax for plane soles and saw tables (to make the wood slide, not to stop rust)
• Clean cotton or microfiber rags
• Rechargeable silica-gel desiccant canisters, or VCI emitters, for the tool chest and drawers
• Evapo-Rust (or a similar water-based soak) for tools that already have rust on them
• A fine abrasive handblock (rubberized abrasive) for light surface rust

Tools

• A digital hygrometer (one in the shop, a cheap one in the tool chest)
• An infrared thermometer to check the iron itself
• A dehumidifier sized for the shop
• A tool chest, till or cabinet with a lid that closes tight

Steps: keep the water off the iron

1. Hang a hygrometer and read it. Know the humidity before you leave for the night. If the shop sits above 50% for days at a time, that's your first job, before any spray can.
2. Close the shop up and dry the air. Shut the doors and windows on muggy days and run the dehumidifier. Empty it regularly, or plumb it to a drain. Fine Woodworking also suggests masonry waterproofing paint on basement walls and sealing garage floors, so moisture can't come up through the concrete.
3. Watch the morning swing. The worst time is a cold night followed by a mild, damp day. Don't throw the garage door open to let the warm air in while the iron's still cold. Shoot the plane sole with the infrared thermometer. If it reads at or below the dew point, keep the tools covered or in the chest until they warm up. Same goes for cranking a heater in a cold shop: the air warms up long before the cast iron does.
4. Wipe every tool before it goes away. Your fingers leave salt and moisture on the steel. A quick wipe with a clean rag takes the prints and the sawdust off. Vic Tesolin points out in Fine Woodworking that dust soaks up moisture and holds it against the steel, which is why old tool chests had a dust seal.
5. Dry it all the way after the waterstones. Sharpening on waterstones (the 10/17 tip) leaves water on the blade, the chipbreaker and the screw. Dry it completely, then oil it. Oil over a wet blade just traps the water.
6. Coat it thin. Hand tools you use every day: a few drops of jojoba or camellia oil, or Moovit, on a rag, wiped on and wiped back off. Cast-iron tops and tools that will sit: a spray that passed the test, like CRC 3-36. Wax on top of that only if you want the wood to slide.
7. Store it closed, with a dry pack. Put a desiccant canister or a VCI emitter in the chest or drawer and keep the lid shut. Fine Woodworking explains that both are made for enclosed spaces: a desiccant fills up and has to be dried out with heat (only some kinds can be), and a VCI gives off a vapor that coats the metal, lasts anywhere from six months to five years depending on the product, and leaks away if the drawer stays open. None of the sprays protect the inside of a power tool, either, so the cases need a dry pack too.
8. Catch it early. A little orange bloom comes off with a fine abrasive handblock; Lie-Nielsen says it uses one in its own shop. Anything heavier goes in an Evapo-Rust bath: the maker says 30 minutes for light rust, up to overnight for heavy. Then rinse with water; the maker calls that step especially important. Let it dry, then coat it.

On the dory: The boat sits where the fog rolls in off the harbor, and the shop sees every swing between a raw November night and a soft, damp morning. So the chisels and the block plane I use every day get a wipe and a few drops of oil when I put them down. The jointer, the bandsaw table and the planes that only come out for the next boat get a coat of something that passed the test. The bronze screws in the planking don't need any of it. And everything that matters lives in a lidded till with a desiccant canister, not out on the bench where the next fog bank can find it.

MOAM's shop note: The most common mistake I see is the guy who waxes his plane soles faithfully and thinks that's rust protection. Wax makes the plane glide; in Fine Woodworking's test it didn't do much to stop rust. The second is the guy who cranks a heater in a freezing garage for a Saturday project. The warm, moist air hits forty-degree cast iron before the iron can catch up, and by Monday the table saw has a rash. Keep the temperature steady if you can, keep the humidity under 50%, and wipe the salt off your tools before you coat them. And if you're near the water, remember: a dry-feeling shop isn't dry enough if there's salt on the steel.

The numbers here come from Fine Woodworking's rust-preventer test, CoatingsPro, the Journal of The Electrochemical Society, the National Weather Service, Woodworker's Journal, and Lie-Nielsen's and Evapo-Rust's own instructions. The dew-point example is my own arithmetic. Read the label on whatever you buy, and test it on a scrap of steel before you trust your good tools to it.

Sources: Fine Woodworking, Thomas McKenna, Got Rust? (July/August 2012); Camellia Oil Outscores Other Rust Preventers; Wax Keeps Rust Off Hand Tools; Vic Tesolin, Keep It Comfortable for You and Your Tools. CoatingsPro (AMPP), How (and Why) to Protect Metallic Surfaces (2012). Schindelholz, Risteen and Kelly, Effect of Relative Humidity on Corrosion of Steel under Sea Salt Aerosol Proxies, Journal of The Electrochemical Society (2014). National Weather Service, Dew Point vs. Humidity. Woodworker's Journal, How Will Cold Air Affect Tool Rusting? Lie-Nielsen, Jojoba Oil and Bench Planes instructions. Evapo-Rust, Four Easy Steps to Rust-Free Tools.

MOAM Out.