How to stop your blades rusting

A craft blade is ground to an edge a few micrometres thick. At that scale rust is not a cosmetic problem that develops over months, it is a performance problem that develops overnight. Long before anything looks orange, the apex of the edge has been eaten into a microscopically jagged line, and the blade that was sharp on Friday drags on Monday.

If you have ever taken a knife out of a drawer and found it worse than you left it, this is what happened.

Why it happens here in particular

Rust needs two things: oxygen and moisture. Oxygen you cannot do anything about, so the entire job is moisture.

Corrosion accelerates sharply once the air is above about 50% relative humidity. In much of Britain that is not a threshold to watch, it is most of the year, and an unheated studio, garage or shed sits above it almost permanently. The practical conclusion is not to monitor the humidity but to assume it is working against you and store accordingly.

Two other causes do more damage than people expect:

Temperature swings. Condensation forms when warm damp air meets a cold surface. Bringing a cold toolbox in from a garage into a warm room puts a film of water onto every blade inside it within seconds. If you store tools somewhere cold, let the box come up to room temperature before you open it.

Your hands. Skin carries salts and acids, and a fingerprint on bare steel is an etched fingerprint a few weeks later. This is the one most people never connect to the rust they find, because the mark appears exactly where the blade was handled.

Carbon steel and the trade you are making

Most precision blades are carbon steel, and that is a deliberate choice: carbon steel takes a finer edge and holds it better than stainless. The cost is that it has almost no corrosion resistance at all. A carbon blade is a sharper tool that needs looking after.

Stainless gives up a little edge quality for a great deal of corrosion resistance, which makes it the sensible choice for anything used around water, stored badly, or handled a lot. The #21 stainless blade is the same shape as the standard fine blade in a steel that will not punish you for leaving it out.

After you finish cutting

Get the residue off. Adhesive, resin, plaster and the acids in some papers all hold moisture against the steel. Wipe the blade before it goes away, not before it comes out next time.

Dry it properly. If you have cleaned with anything wet, including alcohol, let it flash off completely. A blade put away damp is a blade rusting in the dark.

Do not put it away having just handled it. A final wipe takes two seconds and removes the fingerprints that would otherwise etch.

Oil, and which kind

A microscopic film of oil breaks the contact between steel and damp air, and it is the single most effective thing you can do.

A light machine oil is the traditional answer and it works. A rust removing and inhibiting lubricator does both jobs: it lifts existing surface corrosion and leaves a protective film behind, which makes it the right bottle if you are rescuing tools as well as protecting them.

A PTFE or dry film lubricant leaves protection without leaving anything wet. That matters more than it sounds: oil on a blade transfers to the work, and on paper, card or anything you are about to glue or paint, that is a mark you cannot remove. A superfine PTFE precision lubricator is the better choice for blades used on absorbent materials.

Use very little. You want a film, not a coating.

Storage

Keep tools somewhere with a stable temperature rather than somewhere cold, even if the cold place is drier. Swings cause condensation and condensation is worse than steady damp.

A closed container is better than an open rack, and a closed container with a desiccant sachet in it is better again. The sachets that come in packaging are perfectly good for this and can be dried out in a low oven and reused.

Do not store blades touching each other. Contact points trap moisture and damage edges at the same time.

If rust has already started

Surface rust on the flat of a blade will come off, and the rust removing lubricator is made for exactly that. Work along the blade, not across it.

Rust on the cutting edge itself is a different matter. The edge is thin enough that corrosion removes metal rather than sitting on it, so what you have is not a dirty blade but a blunt one with pits in it. Replace it. A blade costs less than the material you will spoil trying to save it.