Rust Preventative Coatings

General, Jul-2020

A few years back, I had some anti-rust coatings left over from various projects so I decided to try an experiment into their effectiveness. I simply applied the coatings to some scrap steel and nailed the samples to a fence post and left them there, checking every now and again for deterioration and taking some photos. The samples were exposed to wind, sun, rain, sleet, snow but were never immersed.

The samples were made with some scrap steel from old TR7 panels. I ground away some of the original paint & primer ready for the coating but left some of the original 30+ year old British Leyland paint finish intact – interesting to compare how this performs to the coatings under test.

The coatings I had available were as follows:

Cavity WaxDinitrol 4941
Rustbusters MIL-SPEC
PaintRustin’s Water-Based Anti-Rust Paint
Hammerite Smooth
Aldi’s Metal Paint
UndersealTetrosyl Tetro Schutz
Dinitrol 3125
TreatmentsRustbusters Phos-Kleen B
Taylor McClure SCALE-X 42% Phosphoric Acid

The treatments are phosphoric acid based solutions that are mainly used to remove rust, clean and prepare the metal for paint but can have some long-term rust prevention properties. Phosphoric acid based products are not a complete rust prevention solution on their own but I included these just to check the extent of any rust inhibitor properties. And in fairness to the cavity waxes, I’d guess they’re not designed to be exposed, so this test is probably not representative of their performance in their intended environment. However, this does seem to be a fair test of underseals and paints.

Here’s  the result after 7 years:

 September 2013 (3 months since start) July 2020 (85 months since start) July 2020 – coatings removed
20132020 test
2013 2020test
 2013 2020rust test

After 3 months, all of the bare metal samples had rusted over and, as expected, the rust penetrated deeper as the years passed. The samples coated with Phos Kleen B seemed to take longer for rust to develop.

After 7 years prior to removal of the coatings, of the paints, the Aldi Metal Paint seems to have deteriorated more than either Hammerite Smooth and Rustin’s Water Based paints – but note that the Aldi paint is around half the price of Hammerite. Rustin’s appears to be the most sound.

All of the cavity waxes seem to have deteriorated badly after 7 years – all had gone hard & almost powdery. But as noted above, this may be an unrepresentative test as they are not normally exposed to all the elements. But it’s worrying nevertheless.

Of the underseals, Dinitrol 3125 seems to have broken down badly in a very similar way as Dinitrol 4941. Pretty shocked by this. Tetrosyl Tetroschutz, on the other hand, still seems sound.

After removing the coatings with thinners the paints appeared to have protected the metal very well. Under the Rustins and Hammerite there was a very feint sign of rust but the metal under the Aldi paint seemed unaffected. Note that the Rustins paint wiped off very easily with thinners. Hammerite and Aldi paints both required a long soak in thinners before the coatings softened and could be removed. The Aldi paint took a bit longer and some vigourous rubbing and seemed more resistant to solvent than Hammerite.

Both cavity waxes were dissappointing – the Dinitrol 4941 had significant rust where the coating was scored or smudged. Note that some coatings were deliberatley scored / smudged (with a fingernail) while still soft – this was to check the claims that the coating would “creep” long after initial aplication to fill gaps & crevices, etc. The MIL-SPEC wax seemed to be generally penetrated across the whole surface of the sample.

The Dintrol 3125 underseal was also dissappointing. You can see a general penetration across almost the whole coated area. Tetrosyl Tertroschutz seemed the best by far – the metal underneath appeared completely unaffacted by rust.