Rust Busters

A little science can help you pick the product that packs the hardest punch.

Rust needs no introduction. Woodworkers with unheated basement and garage shops are all too familiar with this uninvited “guest” that regularly stops by to snack on their shops’ contents. Those working in climate-controlled shops are likely to find rust blooms sprouting from the oils and acids left by fingerprints or born of condensation rings left by a casually-placed drink.

Whether restoring hand tools or reclaiming metal machinery, you have to arm yourself for the fight. The challenge is finding the right weapon. To help you choose, I gathered the top rust busters in the market and conducted my own side-by-side test.

The test revealed that no product is a perfect weapon. Selecting the right product starts by reading the label and understanding some of the science behind a few key rust-busting ingredients. Armed with this knowledge, you’re likely to select a few different strategies. Let’s face it, rust will eventually win the war. But after reading this, you’ll have what it takes to win a few battles.

About The Test

Against every woodworking instinct, I invited rust into my workshop. To do this, I first used acetone to remove any oils or protective coatings from my test tools, and then dipped them into salt water and set them aside to deteriorate.

Two weeks later, I attempted to undo the damage. I treated the tools as specified by each product’s instructions, and periodically inspected the surfaces until the rust was gone. After a neutralizing rinse, I lightly scrubbed the tools with a non-abrasive pad to knock off loose residue.

Choose your weapon

Despite the multitude of products, rust removers can be categorized according to a few key ingredients. Within a category, each treatment works in the same basic manner.

After a quick acid dip, rust revisited a cleaned tool. A longer bath resulted in a protective coating.

Selective chelants remove rust and produce a protective patina, but can compromise good metal.

Electrolysis won’t replace missing metal. Pitted chisels and blades require further smoothing.

Acids

Nearly any acid can remove rust. The process can take from a few minutes to several hours, depending on the acid’s strength and the amount of rust.

Phosphoric acid is the fastest acting, but the least forgiving, especially when cleaning tools with either painted parts or components made from various metals. Phosphoric acid, in liquid or jelly form, is not recommended for use on painted steel, chrome, or even stainless steel. And these acids will quickly dissolve aluminum components.

Acid does not differentiate between rust and good metal. Even milder acidic solutions will etch sound steel. (Under a microscope, etching resembles millions of tiny pits–each a fast-food restaurant for future rust.) Unless dried and oiled immediately after treatment, freshly cleaned surfaces will begin to “flash-rust” within minutes. (See photo above, left).

Phosphoric acid was the only exception to the flash-rusting rule. Leaving a plane blade in phosphoric acid for an hour produced a black, fairly uniform coating of ferric phosphate. This coating can prevent corrosion, but it’ s not as reliable or convenient as other corrosion inhibitors. (See “Keeping Rust at Bay” below)

The greatest dangers relating to acid are to the user. Phosphoric acid can cause burns or blindness upon contact, so gloves and goggles are a must. Kitchen-grade products containing oxalic and citric acids can be used without gloves, but prolonged exposure to bare skin will leave your hands raw.

Selective chelants

Chelants are compounds designed to steal the iron from iron oxide, inactivating it and holding it in solution so that the resulting residue can be rinsed off. This selective process does not affect steel, copper, brass, aluminum, plastic or rubber. All the products listed are non-acidic, water-soluble, biodegradable, and safe to handle without gloves. (The manufacturers don’t divulge the ingredients. I grouped the products together because they perform similarly.)

Unlike the acids, chelants don’t require rinsing; in fact, remaining residue offers some protection (see photo above, center). The solution can be reused, but in time it will leave a darker deposit on your derusted tools. (I save the used stuff for heavy removal and employ a fresh batch for final cleanup.)

Although safer for you and your tools, chelants don’t always discriminate between good and bad metal. In a few instances, the chelants selected sound steel, etching into the lamination boundary lines on my older plane blades, and even creating pits that required grinding.

Electrolysis

This process uses electrical current to convert red rust to black. All you need is an energy source (battery charger), a sacrificial anode (iron scrap), a dip tank, and a tablespoon of baking soda to get the current flowing. Attach the negative lead to the tool, the positive lead to the anode, and plug in the charger. (You can’t see electrons making the jump, but bubbles indicate that the reaction is underway.)

Of the options addressed in this article, electrolysis requires the greatest initial expense. However, once you’ve made the investment, that’s it. And, unlike the other chemical treatments, this plug-powered process is not temperature-sensitive, which is a plus when working in an unheated shop.

Unfortunately, electrolysis isn’t suitable for tools with non-removable wooden or electronic parts. And despite some claims to the contrary, it doesn’t “reverse rust”. Lost steel is lost. When restoring plane and chisel blades, you’ll need to grind or hone any pits down to clean steel.