Keeping bonsai tools from rusting in Florida

Bonsai tools are made of steel, ground thin, and used on wet plant material. In a climate with average humidity above seventy per cent for much of the year, rust is not a possibility to guard against — it is the default outcome of doing nothing.

Why sap is worse than water

Plant sap is mildly acidic and it dries into a hard film. That film holds moisture against the steel and etches the surface underneath.

Pitting from dried sap does more damage to a cutting edge than surface rust does, and it cannot be polished out once it has bitten in. Cleaning sap off is more urgent than drying the tool.

Juniper and pine resin are the worst, and they need a solvent rather than water — isopropyl alcohol on a cloth removes resin without harming the steel.

The five-minute routine

After every session, without exception:

1. Wipe off sap and soil while it is fresh, with a damp cloth. Resin gets alcohol.
2. Dry completely, including the joint. The pivot is where rust starts, because water sits in it and nothing evaporates from inside a joint.
3. Work the joint a few times to move any water out, and dry again.
4. Oil. A light film on the blades and a drop in the pivot. Camellia oil is traditional and non-toxic to plants; any light machine oil works.
5. Store closed, in a roll bag or a box, indoors.

The step people skip is the joint, and the joint is where tools actually seize.

Storage

Indoors, and not in a shed. An unconditioned shed in a Miami summer is a humidity chamber, and tools stored there rust whatever you do to them.

A roll bag is better than a drawer because tools do not touch each other. Loose in a drawer, edges chip against other steel, and chipped edges tear rather than cut.

A silica gel sachet in the roll or box is worth adding — a couple of dollars, replaced yearly, and it changes the microclimate around the tools. Bag them in a sealed container in the wettest months and the sachet does real work.

Never leave a tool outside

Not overnight, not for an hour in the rain, not on the bench under a roof. Dew alone is enough to start surface rust on carbon steel by morning.

Tools left in a pot of water — which people do while working — come out rusting.

Rust that has already started

Surface rust is recoverable. Take it in ascending order of aggression:

1. Oil and a cloth. Light orange bloom comes off with oil and firm rubbing.
2. Fine steel wool or a rust eraser with oil. Work along the blade, not across, and stay off the cutting edge.
3. Fine wet-and-dry paper, 600 grit and upwards, with oil. For heavier patches on flat surfaces.

What not to do: a wire wheel or coarse abrasive, which removes far more steel than the rust did, and any acid rust remover on a bonsai tool, which attacks the temper of the edge.

Pitting that has gone deep is permanent. If it is on the cutting edge, the tool will never cut cleanly again.

Sharpening

Bonsai tools are almost all ground on one side only. Sharpen the bevelled side with a fine stone, following the existing angle, and lay the flat side flat to remove the burr.

Grinding both sides destroys the geometry and is not correctable at home.

Concave and knob cutters are difficult to sharpen properly because of the curve, which is an argument for keeping them clean rather than for learning to grind them.

Stainless is not rust-proof

Stainless steel resists corrosion; it does not ignore it. Stainless left with sap on it in this climate will spot, particularly at the joint and along the seam.

The routine is the same. Stainless just gives you more margin when you skip a day.

Disinfecting between trees

A separate reason to clean tools, and one that matters in a humid climate where fungal and bacterial problems move quickly.

Wipe blades with isopropyl alcohol between trees, and always after cutting anything that looked diseased. Alcohol evaporates, so oil afterwards.

Bleach solutions work and corrode steel. Alcohol is the better choice for tools you care about.

A note on new tools

Many tools ship with a protective coating or oil film. Clean it off before first use, then apply your own oil. Some factory coatings are not something you want on a fresh cut.

Where to look

Tool sets arriving with a roll bag or a wooden box — which is the storage as much as the packaging — are on the bonsai tool sets page. Individual cutters, shears and scissors are at cutters and shears, wire and wire cutters at training wire, and the repotting tools at tweezers, rakes and repotting.