Tannic Acid vs Phosphoric Acid Rust Converter

Almost every rust converter on the shelf is built on one of two chemistries: tannic acid or phosphoric acid. Both convert rust in place so you don't have to grind back to bare metal, but they do it differently, and each has jobs it suits better. We make Rust Halt™ Rust Converter, which is a phosphoric complex, so we've tried to keep this fair. Here's the plain-English difference.

What does a rust converter actually do?

Rust is iron oxide. It's porous and flaky, it holds water against the steel, and it keeps growing. A rust converter reacts with it chemically and turns it into a different compound that's stable, doesn't hold water the same way, and gives paint something to bond to. The two main chemistries just produce different compounds.

How tannic acid rust converters work

Tannic acid is a plant-derived compound, the same family of chemicals found in tree bark and tea. It reacts with iron oxide to form iron tannate, a dark blue-black layer. Many tannic converters also contain a polymer or latex binder, so they dry to a film that works as a primer.

• They're generally milder than acid-based converters.
• They work well over thicker, well-attached rust.
• The polymer film means many of them act as a primer straight away.
• They tend to need a fair amount of rust to react with, and don't do much on lightly rusted or bright metal.

How phosphoric acid rust converters work

Phosphoric acid reacts with iron oxide to form iron phosphate, a hard, stable, dark layer. It also lightly etches the surrounding steel, which gives paint a very good key. It's the same chemistry behind the phosphate pre-treatments used on steel before it's painted in industry.

• It works well on light to medium rust and pitted steel.
• Loose surface rust is partly dissolved rather than just coated.
• The etched, phosphated surface is an excellent base for paint.
• It's an acid, so it needs keeping off chrome, aluminium, stone and good paintwork.

Side by side

Where Rust Halt fits

Rust Halt is a phosphoric converter, but it isn't just phosphoric acid in water. It's a phosphoric complex: we process it with ultrasonic technology so the active phosphoric acid is held inside surfactant-coated droplets, a micro-emulsion. Those droplets wet into pits and seams, cling to vertical steel, and release the acid where it meets the rust, so it converts evenly instead of running off.

In use, the rust turns black in about 20 minutes, and you can paint over it after 3 hours. Rust Halt converts rust into a primed, paint-ready surface, and it cleans up with water.

Which one should you choose?

• Light to medium rust, pitted steel, gates, railings, chassis and bodywork: a phosphoric converter is a strong choice, especially where you want a hard, well-keyed base for paint.
• Thick, heavily rusted steel you can't brush back: a tannic converter can suit, though thick loose scale should still come off first, whatever you use.
• Metal that's rusted through: neither. Cut it out and replace it.
• Aluminium, stainless, chrome or galvanised: neither. Rust converters are for iron and steel only.

Frequently asked questions

Is phosphoric acid or tannic acid better for rust?
Neither is better for every job. Phosphoric converters suit light to medium rust and give paint a strong key; tannic converters suit heavier, well-attached rust.

Do both types turn rust black?
Both darken the rust as they convert it. Tannic converters usually go blue-black; phosphoric ones dark grey to black. Rust Halt turns rust black.

Can I paint over both?
Yes. Both are designed to be painted over, and both should be, because a converted surface on its own isn't a finished coating.

Can I use a rust converter on galvanised steel?
No. Rust converters are for iron and steel. Galvanised, aluminium, stainless and chrome surfaces need different treatment.