Phosphoric acid for treating rust

[Thamesbank]

I know I should sand blast the bloody hull, but I can't afford it. So...I bought some phosphoric acid solution. Now this stuff is pretty concentrated. I mean it seems dangerous to be near the stuff.
My question: Am I mad to use this on my underwater hull? Having seen what this did to one of my rusty garden tools I guess I should know the answer already: Keep the lid on the tin and just paint lots of epoxy all over the lot and pray it doesn't all fall off.
The acid does not come with any detailed instructions, just the name "Rust converter" so I guess I have to wash the damned stuff off/neutralize with an alkali before painting.

 

[Thamesbank]

Update on the experiment

I left the acid on a saw blade for 30 mins and the result? Echted steel where before it was shiny with the rust becoming covered with what I assume is iron phosphate. Not really the results I had hoped for.
Anyone need 5 liters of concentrated phosphoric acid? Useful for removing garden tools I reckon..

 

[Pasarell]

Re: Update on the experiment

Good decision. In the coatings industry we find rust converters more trouble than they are worth.
There's no substitute for some form of blasting on steel - and if that's not possible grinding is second best.

 

[Thamesbank]

Bumping and grinding

Thanks for the feedback. I guess by grinding you mean really removing back to new steel. I have taken the surface back with 24-grit sanding disks on an angle grinder, then with a steel brush (also on the grinder) and to finish an attack with an excentic sander with a 40 grit disk. The result: The rust is still there, if only as a redish tinge on top of the shiny steel. Still I must say that my forearms are now really strong!
Question for me now is: Is this sufficiently clean for epoxy or shall I just throw bitumen all over the damned boat?

 

[pappaecho]

Re: Bumping and grinding

I was rather under the impression that phosphate etching of iron was to give a sealed layer (=oxygen free) to prevent further corrosion and hence a stable base onto which other treatment such as epoxy could be applied

 

[ccscott49]

We use "Ospho" rust treater (phosphoric) here in the offshore oilfield, works very well, clean up the steel best you can (you've already done that) treat with Phosphoric, wash off, dry, paint! Works for us for many years.

 

[Bajansailor]

I will endorse Ospho - marvellous stuff!
If you cannot blast the steel, then cleaning and Osphoing it is probably the next best way you can prepare it prior to painting.

 

[Pasarell]

Re: Bumping and grinding

Grinding does remove the corrosion but it's disadvantage compared to blasting is that it doesn't clean out the surface pits effectively. Disk grinding is best but you have to distinguish between removing the corroded steel and polishing it. Wire brushing particularly can give a very shiny surface but that is actually polished rust!
What you coat with afterwards depends on how you want it to look. Bitumen is old fashioned but effective provided you don't want it to look good or overpaint it later.
Surface tolerant epoxies will perform well if the preparation is pretty good
I think moisture cured urethanes are best for a variety of reasons but I have to declare a commercial interest at this point!

 

[AndrewB]

Although it doesn't say so on the tin, it is definitely a good idea to remove the converted rust before painting. Any residual traces of acid will eventually undermine and spoil the paint job. A thin layer of converted rust can be removed by wire brushing, which gets down into pits without needing grinding. But as Pasarell says, beware of burnishing any remaining unconverted rust. (Burnished rust presents a blue-grey shiny surface, darker than the colour of steel. It is resistant to acid converters and will need to be ground out).

 

[Jake]

Hi Thamesbank.

I've been told to get hold of some Phosphoric acid for prepping a just-blasted ferro-cement hull for epoxy. It neutralises the lime, and tackles some of the rust bleeds, I'm told. What was your source for the stuff, and how expensive was it?

 

[Thamesbank]

Hi Jake,
I found the stuff through a German retailer on ebay. He charged me about 35 quid for 5 litres, so it was cheap (ish).