Just a few comments. Phosphoric acid isn't a rust converter, at least
not in the sense described below. Phosphoric acid, and in fact most
acids, will actually remove rust. The acid reacts with the oxygen in
the oxide and frees it. If you check the ingredients of products that
remove rust, like Metalprep and Naval Jelly, you'll find that they
contain phosphoric acid. Phosphoric acid also has a secondary reaction
which leaves a coating of iron phosphate on the surface.
Remember the old legend about Coca Cola being a good rust remover?
Check the ingredients next time you have a Coke!
The advantages of phosphoric acid are 1) The reaction with the rust is
much slower than its reaction with the surrounding iron (which it also
eats). 2) it leaves a paint friendly coating of iron phosphate behind.
It also has some disadvantages. 1) The reaction is very slow, so unless
the surface has only light rust, some will remain under the coating. 2)
Phosphoric acid isn't solder or weld friendly, so if you plan do do
either, do them first if, possible.
You can use other types of acids for quicker rust removal. e.g. HCl.
The big problems with HCl are 1) It's highly corrosive, so its hard to
work with. You'll need face and body protection. 2) It releases
chlorine gas which not only smells bad, but is also highly corrosive.
Chlorine gas is a very strong oxidizer, so any iron based metals in the
vicinity will rust almost instantly. This goes for the surface being
treated, BTW, so instant cleaning and painting is necessary. The
chlorine reaction is somewhat similar to rust caused by road salt, which
also contains chlorine. 3) Its reaction with the surrounding metal
(i.e. etching) is about as fast as it is on the oxide. In short, I
don't recommend HCl for this purpose, except under very restricted
circumstances. I don't think the underside of a car is such a
circumstance. A wire brush is probably the best approach here, and
phosphoric acid, or something similar afterwards. However, don't use
acid on suspension parts, springs, etc. because the acid treatment will
weaken them due to hydrogen embrittlement.
I've used so called rust converters, but haven't been very impressed.
If you scape the "converted" coating off, generally you'll find rust
underneath. It only seems to work on the top most layer, so you'll
likely have to remove all visible rust before applying it to get the
results claimed.
I've also used POR15, but mainly in place of regular paint. It hasn't
been on long enough for me to make a judgement call yet, though. I do
like the fact that it seems to flow into a smooth coat when applied
with a brush or one of those sponge applicators to form a nice even
surface.
Roland
> ZinkZ10C@aol.com wrote :
>
> > For various projects I use "Extend" brand rust converter by Permatex. It
> > is/contains Phosphoric acid to convert the rust to a more stable
> > compound.
>
> Permatex Extend (used to be Duro Extend) is a lot different than the old
> phosphoric acid "Naval Jelly". It has some sort of catalytic reaction with
> the rust, and in fact will not turn black if the surface is completely free
> of rust. One of the hazards of using the liquid is that if you stick a
> brush back into the bottle after brushing it on the surface, it may
> contaminate the bottle with already catalyzed liquid, which will eventually
> convert the entire bottle.
>
> I've been using a very similar product called Klean-Strip Rust Converter.
> It's main advantage is that it's available at Wal-Mart (with the Bondo and
> whatnot).
>
> Randall
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