Derusting with phosphoric acid

[Skillster]

Am in the middle of prepping the front cross member of my MGB and could do with some advice please.

I have got the thing cleaned up and the major rust build-up removed using the electrolytic method (battery charger, bag of washing soda, wheelie bin etc.)

After drying the crossmemebr by the boiler for a couple of days I lightly brushed / sanded to remove the new 'flash' rust and deal with some areas where the scale hadn't dropped away.

As a final step I brushed on some phosphoric acid top kill every last bit of rust and convert it to phosphate. The acid was 40% concentration from a farmer's supply shop; I brush loads on and wiped off as best I could, but the metal remains coated in a sticky layer of the acid and doesn't show any signs of drying.

How can I deal with this (assuming I need to)? I will be painting with acid etch and then Tetrosyl Chassis paint. I assume that washing the sticky acid residue off with water will completely defeat the object?

Help!

 

[Omniata]

You need to wash it thoroughly after using acid cleaners...

Dry with kitchen paper then bring it inside, use a little wet and dry if necessary to finish and that should be fine...

Maybe give it a wipe with solvent before painting...


Just make sure you get all the acid off...

 

[Skillster]

Thanks for that. I'll give it rinse and get the hot air-gun on it.

I think I had a notion that given I was using acid etch primer anyway, a bit more phosphoric acid wouldn't hurt. Plus I remember using Jenolite (just phos. acid in a gel form?) that you just allowed to dry before overpainting.

 

[rikrobson]

You could use a solution of the washing soda to neutralise the acid. The rinse and dry.

I would have left the electrolytic process run a bit longer. It Won't harm good metal. I've left bit is for several days and the longer the part stays in the better.

 

[casement]

Phosphoric acid is good stuff, but don't use it for anything other than steel.

Many years ago I dipped a couple of Amal carb bodies and partially dissolved them.

 

[Skillster]

You could use a solution of the washing soda to neutralise the acid. The rinse and dry.

I would have left the electrolytic process run a bit longer. It Won't harm good metal. I've left bit is for several days and the longer the part stays in the better.

It was in the wheelie bin for the best part of 10 days; the technique works really well and IMO is much easier than blasting (well it is if you don't have a blaster!). The thing was that in places the surface 'crust' need some persuasion to come away (leaving the shiny pitted steel underneath).

Using the phosphoric acid was a final 'belt and braces' to convert the new flash rust and (in theory at least) some of the other black oxides to phosphate, essentially to give the acid etch a flying start.

I'll use a sodium carb. solution as a rinse, as you suggest, before getting the hot air gun out to try and dry it as quick as possible.

As I said, all the commercial products using phosphoric acid that I have encountered over the years (Jenolite etc) don't include rinsing in their instructions but simply require wiping and time to dry; anyone care to suggest why?

Thanks,
Duncan

 

[metalspanner]

I have done the acid bath thing using the acid from old batteries, and what I do to clean off and neutralise the acid is run boiling hot water over it, it has to be boiling though, this clears all the acid off, and as a bonus leaves it bone dry to boot.
Cold water will just leave a messy residue that needs other solvents to remove.

 

[eil]

As I understand you need to neutralize Hydrochloride and Sulphuric, but you do not want to with phosphoric, you just let it dry and remove the loose white powder with wire brush or scotchbrite,wash down with solvent and then paint, there is some prep info before epoxy at the bottom of the page on this link http://www.epoxysystems.com/surfaceprep.aspx

 

[Skillster]

As I understand you need to neutralize Hydrochloride and Sulphuric, but you do not want to with phosphoric, you just let it dry and remove the loose white powder with wire brush or scotchbrite,wash down with solvent and then paint, there is some prep info before epoxy at the bottom of the page on this link http://www.epoxysystems.com/surfaceprep.aspx

Interesting. Here is an extract of what these guys say:

Always use a 10% solution of Phosphoric Acid.
Apply phosphoric acid solution by either paint brush or rubber squeegee and allow metal surface to AIR DRY.
DO NOT FLUSH METAL SURFACE WITH WATER

I used 40% acid straight from the container and it hasn't dried. I wonder now if I should have used a dilute solution (10% as it says here?). I think as a next step I'll try using a rinse solution of dilute phosphoric and see what happens.

Surely things shouldn't be this difficult!!!

Thanks.