A technique for removing rust is etching with Phosphoric Acid. Phosphoric
Acid has a unique property of dissolving iron oxide quickly while etching
iron very slowly. This means that you can leave metal in Phosphoric Acid
for much longer than necessary with very little damage. The acid will
attack bare metal slowly and will start the process of hydrogen
embrittlement, so use the minimum etch time that removes all rust.
Another unique advantage of Phosphoric Acid is that it leaves a fine
coating of iron phosphate behind. Iron phosphate prevents rust. However,
the iron phosphate coating is not very thick and not durable. Some
additional protection is still required.
Phosphoric Acid etch will leave a hard, bright metal finish. This is
because it will etch the surface slightly, exposing new, bare metal. Often
this is desirable. It leaves an attractive surface and a surface ready to
paint. A common product which contains Phosphoric Acid is Naval Jelly.
Auto body shops treat metal with acid metal wash, a solution of Phosphoric
Acid and alcohol, before painting. This removes waxes and oils, removes
slight amounts of rust that form between sand blasting and painting, and
leaves a thin protective coat of iron phosphate. One commercial solution
for this is DuPont Quick-Prep. Sherwin Williams has a similar product
called Metal Prep.
One more technique for removing rust is Electrolytic Rust Removal. Rust can
be electrically etched off of iron or steel in a bath of mild alkali, such
as Sodium Carbonate (swimming pool supply stores. Ask for product to raise
water pH). Connect the rusty part to the negative terminal of a 12V battery
or battery charger and a scrap piece of steel or iron to the positive
terminal. Use one tablespoon of Sodium Carbonate per gallon of water. This
technique has many advantages. First, the alkaline solution is much safer
than some of the acids mentioned above. It is still a chemical, so rubber
gloves and splash goggles are recommended. Another advantage of
electrolytic rust removal is that it will have no effect on good metal, so
you can leave the work in the bath for a long time and not damage the
metal. There is no risk of hydrogen embrittlement nor of etching unrusted
metal.
Electrolytic rust removal will leave a black oxide surface, which is the
result of a process that doesn't remove any good metal at all. The only
thing removed is loose rust and embedded oxygen. If your goal is to keep as
much of the original metal behind, this is the best technique.
After removing ALL of the rust, I prep and spray immediately with a
zinc-based primer, followed by a regular red primer, followed by the final
color. Allow plenty of time (days, not hours) in between coats, or you'll
be dealing with soft paint while you're trying to re-assemble parts. Powder
coating, of course, is nice if you really want to go whole hog.
- James Einolf
'74 MGB (almost finished replacing the rear brakes that rusted solid and
were *disconnected* by the DPO - without telling me when I bought it, of
course....)
>I was wondering what is the best way to prepare and paint parts.
>Over the next couple of weeks I will be redoing my front suspension.
>While I have it apart I would like to paint the A-arms, kinpins,
>radiator shroud, etc. All of the following would address both
>painting with an air compressor and without an air compressor. Currently
>I don't have one but I will be buying one in the next couple of months.
>
>1. What is the best way to prepare the surface?
>2. What type of primer and paint?
>3. What is the most durable?
>4. Should special paint be used for higher temp. items such as
> radiator shrouds, coolant overflow containers, etc.
>
>I was just going to clean off all of the grease, remove the loose paint
>and rust with a wire brush, and spray it with a can for rust-o-lem (sp?)
>black. Am I missing anything? My main concern is that I spend time
>doing this and then the paint chips or peels off in a short period of time.
>Any advice?
>
>Thanks,
>
>Bill Gilroy
>77 Midget
>wmgilroy@lucent.com
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