Rust converters. Any use?

[pcatterall]

When messing about with classic cars and boat bits I have often used these converters. I have read many times on this forum that they are of little use. I always try to remove loose rust and all rust where I can 'get at it' . When I see any remaining rust turn black and acquiring that 'treated look' I always feel that I have done something worth while. What's the real technical story?

 

[RichardS]

It is certainly worth doing, however, it must be followed before too long depending upon conditions, with a proper protective layer of primer/paint/sealant or the rust will return albeit delayed compared to doing nothing.

Richard

 

[rogerthebodger]

I have given up on rust converter and use hydrochloric acid until you get to a uniform grey colour followed phosphoric acid wash and dry then paint with high build epoxy

 

[Lightwave395]

I have given up on rust converter and use hydrochloric acid until you get to a uniform grey colour followed phosphoric acid wash and dry then paint with high build epoxy

I stopped buying 'rust covertor' long ago at £10 for a thimbleful and bought a 5 litre container of phosphoric acid for a tiny fraction of the cost. Used it now for a few years and it seems to work well

 

[JumbleDuck]

The worst results I ever had were with Fertan on the bilge plate/washer which supported my Jouster keel. I had the keel off so I wire brushed it then used Fertan as directed, then painted assiduously. Rust all over the place in a year. Cleaned it up (again), gave it two coats of Finnegan's No 1 - now Hammerite No. 1 - and one of Danboline and this is how it looked 23 years later when I sold the boat. No further treatment, cleaning or painting:

 

[Skylark]

I’ve used phosphoric acid once or twice with mixed results. I’d be interested to read the views of our resident chemists/metallurgists/alchemists.

For light work, a quick application followed by wash, dry and primer/undercoat/topcoats may give the desired results but for anything larger I’d favour an angle grinder.

 

[Stemar]

gave it two coats of Finnegan's No 1 - now Hammerite No. 1 - and one of Danboline and this is how it looked 23 years later when I sold the boat. No further treatment, cleaning or painting:

Unfortunately, the recipe's change, presumably courtesy of elfin safety, so Hammerite just isn't the same now.

 

[rogerthebodger]

I’ve used phosphoric acid once or twice with mixed results. I’d be interested to read the views of our resident chemists/metallurgists/alchemists.

For light work, a quick application followed by wash, dry and primer/undercoat/topcoats may give the desired results but for anything larger I’d favour an angle grinder.

I don't consider phosphoric acid to be strong enough to completely remove all the rust.

This why I needle descale then use hydrochloric acid until back to grey metal. Phosphoric acid will remove the flash rustion you will get after the hydrochloric acid and does leave a protective layer to stop rusting until you can get a proper epoxy coating.

Grinding will just spray the rust and any metal all over the place to spread the rust.

The other issue is any rust left under the edges of the paint next to the rusty metal. This can also start the rust again.

 

[DinghyMan]

I've found Bilt Hamber Hydrate 80 to be good stuff on rust or scale Bilt Hamber Hydrate 80 (500ml): Amazon.co.uk: Car & Motorbike

Like most it needs surface scale or rust to work, I used it for test piece of garden steel work I did for SWMBO and two years later and its still looking good.

Paint it on, wait 20 mins or so, and it turns black, its high build and seems to take almost any paint without any issue so far

 

[DownWest]

I read in Which quite afew years ago, a test on converters and Hammerite (before they changed the formula) and they weren't very impressed. Grinding back to bright metal was best. Though I would go with Roger's acid to grey metal.