Surface rust that wipes off is cosmetic, pitting matters only on thin tools such as blades and springs, and rust that has eaten through the cross-section where the tool takes its load means the tool will fail and should be left. Confirm soundness by flexing the tool, tapping it and listening for a clear ring, and reject any cracked grinding wheel, mushroomed striking face or chipped chisel head, because fragments from those are thrown.
Part of Tool Identification & Repair
The metal check is the most important of the three field checks, because a tool fails at the place where force goes through it.
Three Kinds Of Rust Need Three Different Responses
Surface rust is an orange-brown coating that wipes or scrapes off to solid metal underneath. It does not weaken a tool, and almost every unprotected steel tool left in the weather has it (parent article). Take the tool.
Pitting rust has eaten small holes into the surface. Run a fingernail over it: distinct craters mean metal has been lost. For thick tools such as wrenches, crowbars and hammers, pitting is cosmetic. For thin tools such as saw blades, knife blades and springs it weakens them. Take thick tools and judge thin ones one by one.
Structural rust has eaten through the cross-section: the metal is flaky or crumbly, or has visible holes. In a load-bearing area, such as a hammer eye, wrench jaw or plier pivot, the tool will fail under stress. Leave it unless you can cut off the damaged section.
What You Need
- A good light, a nail or awl for feeling the surface and a wire brush to expose bare metal.
- A small hammer and a length of string or wire for the ring test.
- Eye protection and gloves.
Method: Look, Flex, Tap
Step 1: Brush the suspect area down to bare metal, so you see the section and not the crust.
Step 2: Flex test. The parent article’s test is to grip the tool and flex it. If it moves or crumbles where it should not, it is structurally compromised. A wrench that twists under torque is useless; a screwdriver shaft rusted thin will snap. Do this with the tool held close to your body and your face turned away, in case it breaks.
Step 3: Tap and listen. Hang a small tool from a string, or hold it loosely at one end, and tap it lightly with a hammer. A sound piece rings clearly; a cracked piece gives a dull thud.
Step 4: Look for cracks. Smear a little oil on the surface, wipe it off, and dust with chalk or fine ash; oil seeping out of a hairline crack stains the dust. Check inside corners, at the eye of a hammer and at the root of a jaw.
Step 5: Check for heat damage. A tool that has been in a fire may show blue or rainbow colours and may have lost its hardness. Edges that have gone soft will not hold a sharpening; test with a file as on Recognizing Improvised Tool Sources.
Step 6: Check the working face. A mushroomed hammer face or chisel head has spread metal that can chip off; dress it on a grinder or file, or retire the tool. A chipped chisel edge can be reground. A cracked or chipped grinding wheel is scrap.
Common Mistakes
- Rejecting a tool for surface rust. The rust comes off with the methods on Rust Removal & Prevention.
- Trusting the outside of a rusted tube or bar. The inside may have gone.
- Hammering a hardened tool “to test it.” Hardened steel can shatter.
- Using an old file or spring as a lever. It is hard and breaks instead of bending.
Safety
Cracked wheels burst, mushroomed faces chip and stressed steel snaps
A grinding wheel with a crack can burst at speed and throw fragments, so ring-test every wheel by tapping it lightly while it hangs free and reject any that sounds dull; also never exceed the speed marked on it and stand to one side when starting it. A mushroomed hammer or chisel head throws fragments when struck. Wear eye and face protection for grinding, striking and flex tests. Do not use a tool with a hairline crack in a load-bearing area.
What’s Next
- Learn next: Rust Removal & Prevention — recovering tools that pass the check
- Combine with: Grinding Wheels — wheel care and use in detail