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phosphoric acid on bare bonnet?


molten
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Hello

I have stripped my 1500 spit bonnet to bare metal on top (left underside to prime over). Some previous filling had been done as some lazy toad could not be bothered taking small dents out with dolly.
Anyhooo, done some metal work on the arches and now preping ready to paint.

The top of the bonnet (after being stripped with nitromors) seems to have a dull matt finish to bare metal.
Did triumph galvanise or dip bare steel bonnets in some form or metal protection?

I have applied some 40% phosphoric acid solution to one wing and a few hours later removed the dry residue with wire wool. Looks ok but seems to remove the dull matt finish mentioned above.
I plan to hit the entire top surface of bonnet with phos acid to etch for primer.

Can you spot any issues here?
Cheers

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You do need to wash the HPO3 off with water when the reaction with rust is complete.
Phosphoric acid continues to react with steel, not just iron oxide.

I unfortunately know because I left some small steel parts in a container of a typical liquid phosphoric acid rust remover product for several days, erroneously thinking there was no need to monitor it as the reaction would end when the iron oxide was gone.  The phosphoric acid variously ate through, pitted, severely diminished, or obliterated the parts.

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rotoflex wrote:

I unfortunately know because I left some small steel parts in a container of a typical liquid phosphoric acid rust remover product for several days, erroneously thinking there was no need to monitor it as the reaction would end when the iron oxide was gone.  The phosphoric acid variously ate through, pitted, severely diminished, or obliterated the parts.


Ditto with me! I left some grub screws for a lathe in a rust remover product for a week and when I wire brushed them clean the threads brushed off!

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Water is recommended to remove the Phosy reside as that is its solvent. Acetone may not work - worth a try though.
Having cleaned the surface with the phosy it will be protected from a short dampening of the surface.
As mentioned total immersion is quite different from a surface prep. As the Phosy is being used it is being neutralised. In a bucket there is plenty more to take over whereas a surface prep has a limited amout.
EG. etch primer stops working fairly quickly as there is only a small amount.

Roger

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Hi Roger

My phosy solution is 40%. I have been applying neat to bare metal and the dry residue is then removed with wire wool. Do you think this will need neutralising? Perhaps not as indicated above.
Alkaline solution (soda crystals) can be used to wipe off but then another wire wool rub to remove that, but i dont want to put any more water based solution on the treated steel.

I have also noted that the area I did yesterday has now started to turn yellow/brown. Likely rust flash. I never dried thouroughly enough yesterday. So will use paper towels & compressed air next time. Top side of bonnet is now 99.9% free of any paint. Only a few small minor rust pitting spots and phosy seems to have dealt with them. Will get back to it tomorrow aft.
Thanks

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Been using Phosphoric acid as Jenolite/Janitol/Altrans from Deb (bought by the 1 gallon) for some 45 years.  Best derusting agent there is if used appropriately.

When soaking rusted badly components the problem you get is that the rust has gone into tiny deep pits and if you leave it to soak in the Phosy acid for a long time it will simply keep on working away until it has removed all the rust - and left deep craters as I found to my cost with some rare special components!  Simply keep and eye on the component being derusted and get it out when ready.  Actually parts with just a surface rust can be left in solution for a very long time because when derusted all further action stops.

On panels I wire wool the Phosy acid over the surface and leave to work for a while, repeating if necessary.  If there are pitted bits I work the phosy acid in with a wire brush (rotary brush in a drill on slow speed) - usually using the gel form which I buy in 1 litre containers as Evans Hi-Phos - both Altrans and Hi Phos can be bought from janitorial suppliers.

Heavily rusted spots that the gel cannot remove and leave clean metal turn black which after a couple of goes. I wipe off the gel and scrape the black surface with a scriber/nail to make sure that there is no brown rust under the black phosphate layer - if there is I apply more Hi Phos and use the scriber/nail to scratch the black surface so that the gel can get at the rust and react with it.  This is great for dealing with those annoying pitted patches that will go paper thin if you keep rubbing them down (this method leaves thin patches but the area retains its general strength).

Although the manufacturer says wash off the phosy acid and dry thoroughly I prefer not to use water but to wipe it off with industrial paper towel (dampened if necessary) and rub away until all the stickiness has gone.  This leaves a nicely protected panel and avoids flash rusting.

Don't forget that when handling phosy acid the use of gloves is essential - and preferably eye protection and some old overalls/cloths.

For priming I normally use a standard etch primer but will probably try a wash primer next time (as recommended by someone on the forum) which contains phosphoric acid, as extra protection and also for etching.

Ted

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A method I used with success on rust pitting on the bonnet after trying for a while with phosphoric acid and a fine stainless wire brush, was to make a pad of Scotchbrite  soaked in washing soda solution wrapped around a piece of sheet lead connected to the positive of the battery.  The bonnet was still on the car, so connected to the negative.
Had been using electrolytic rust removal for a while on small parts so figured this might work.
May have been wise to wear rubber gloves as the tingling in the fingers, probably due to small cuts, was rather stimulating.
Quick and good result in this case.

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Hi Ted,
           the standard 'etch' primer does contain phosphoric acid as that is the etchant.
Interestingly, the etchant isn't there to stop/remove rusting but to ensure that the primer is 'stuck' to the surface. Very important on reactive surfaces such as Aluminium and its alloys.

Roger

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As an aside, if you have access to an Agricultural Merchant in your area, Phosphoric Acid can be bought cheaply as Milkstone Remover. This is intended for removing calcium deposits from milking machinery, it needs to be substantially diluted, at least 50:50, to bring it down to the strength of the best commercial rust removers. It also includes an indicator dye so you know when the solution has finished working.

I last paid around £20 for a gallon, but my supplier has recently switched to 25 litre containers for only a little more money. It means I have to take the car with me to collect though......

Cheers,
Bill.

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Useful info Bill

I normally pay £20 to £25 per 5 litres at our local janitorial suppliers.  I notice by checking the web that milkstone remover can come out at £17.50 so will check our local branch of Countrywide to see if they do it.

For general info, Altrans is sold for rust removal, aluminium cleaning (not anodised), and in janitorial use as a calcium/lime removal agent in toilets.

Don't forget that citric acid is a great remover of rust (100g to the gallon) and is absolutely safe to handle.  You can put your bare hands (provided skin is not broken) in a bath of the stuff to retrieve components though rinse your hands afterwards with clean water.  If you get some in an open wound it does not really do any damage - just stings like hell!  You can also chuck waste stuff down the sink quite legally which you cannot do with phosy acid!  The big drawback is that the surface flash rusts and needs a light treatment with something like phosy acid to stop it.  It is also very cheap to buy.

Ted

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The point of phosphoric acid, unlike other organic acids like citric, is that iron phosphate is relatively insoluble. It dries to a hard rough surface ideal for painting.    Washing off steel treated with citric will remove the iron citrate, and washing off excess phophoric acid is probably no harm.  As for neutralising it, the chemistry gets complex and you might remove the phosphate.    Wipe if you must but I prefer to just let it dry.

It is, shall we say, mistaken thinking to leave any metal parts in an acid bath longer than they need to be there.   Iron oxide is more vulnerable to acids than the native metal, but not so much that it won't be eroded.  The weak, organic acids widen that window so that they will attack the oxide in preference to the metal, but they are not intelligent enough to stop when that is done!

John

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  • 10 years later...

So pleased I searched for this to check! I've just treated some surface-rust patches on my fuel tank (water was dripping on it over winter from condensation as my boot boards were removed) and was tempted to leave the hardened-gel on it until next weekend. Likely it would have neutralised anyway, but I certainly don't want to pitt/weaken/thin the fuel tank. 

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Some paint makers advise against using any etching products under their primers.  SPI 2K epoxy primer, for example.  Evercoat says you can use etching products under their 2K Featherfill primer, but it extends the cure time about 5X.

Ed

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Hi Ben,

rather than remove the rust why not use one of the rust Converters (Kurust etc)

Remove the excess rust with emery etc and then apply the Converter, allow to dry the apply primer.

Also, don't forget that you can't remove ALL the rust. What is on the surface is easy to see but the intergranular corrosion keeps on going.  It can be slowed down dramatically  -   but not removed.

 

Roger

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