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Old 04-18-2017, 05:23 AM   #5
1project2many
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Join Date: Mar 2012
Location: Lakes Region NH
Posts: 3,149
Re: Finishing a body repair

Treating rust really takes a good understanding of what you're fighting. Rust that we're fighting is the result of iron reacting with air in the presence of moisture.

Although people think of rust like a cancer, it really should be thought of as a scab over a wound. Removing all of the rust in an area doesn't heal the metal underneath, but leaves the metal exposed and ready to scab over again. Removing rust to allow welding or to allow a coating to adhere to the base metal makes sense. But if you can remove any moisture from the rust then seal the area and the metal underneath it away from water and air, you will successfully prevent future rust.

Rust converter is a good product if used properly. Marketing that says it works miracles is garbage. Rust converter is primarily phosphoric acid which can chemically change rust to a form of iron oxide which is not so porous and is less reactive. The end result is not a sealer in any way, and the conversion process is only a few mils deep so rust converter is not to be used instead of many hours of rust cleanup. It is good for areas where its difficult to get into to assist in cleanup.

Most efforts we can make to slow rust involve some method of sealing the metal away from air and water. Epoxy paint is popular because it will harden and cure in a way that can prevent exposure even if it doesn't bond properly. Single stage polyurethane can be effective also. Waxes and oils are generally less effective over long term. No sealer will work if only one side of a piece of steel is protected.

Weldable primers apply a coating, primarily zinc, which tends to react with oxygen faster than iron. It is intended to replace factory applied galvanizing when repairing panels. The primer coating will get "used up" over time leaving the iron below exposed. It is made to be used between panels that are spot welded together. It is not made to be welded using an exposed arc as that process burns up the coating. Arc, mig, or tig welding a zinc based weldable primer produces toxic gas. Bad juju. Weldable primer is not a complete solution. It is "better than nothing" and it will work with other products.

Galvanizing is the process of electrically applying zinc to steel to provide a sacrificial coating. There are "cold galvanizing" sprays which are used to apply a heavy layer of zinc to steel after a repair. There are also some methods of galvanizing small pieces of steel at home, but it's not really for the hom hobbyist. Like weldable primer, zinc applied in galvanizing is a temporary protection (although it can last for many years) and will eventually get "used up." Also like weldable primer, welding through galvanized steel without removing the zinc creates toxic gas.

Unlike iron, the green copper oxides that form in air will create a barrier that prevents additional oxidizing. It is expensive but it can be very effective. Copper can be bonded to steel through electroplating, or it can be included in some coatings as a preventative. Copper was more popular many years ago due to lower cost.

OE answers for the areas where steel was butted or overlapped then welded for generally involved treating the area after welding occurred. Often a body would be submerged in liquid then electrically charged to encourage the rust protection to adhere to areas that spraying would be difficult to cover. This is extremely difficult to duplicate for a home hobbyist.

These days, many OE pieces are put together with adhesives. Adhesives can act as a sealer, can be applied over some coatings, and do not have the effect of heating, warping, or changing the steel to which they are applied. They are often stronger than the welds they replace.

It is sometimes practical to design your repair to use stainless in areas where it is most difficult to treat and to overlap this material with mild steel in places where both sides are easily accessed. Message board knowledge often tells us that steel and stainless can't be welded, but these days there are so many versions of high nickel, lower corrosion steel available that as a blanket statement it is not correct to say they can't be bonded. However, using a mild steel wire or rod with stainless will decrease its ability to resist corrosion.

All of these options and answers are also not an answer. Why? Because there is no single best way to prevent rust. Folks in sunny southern CA can get away with a much different strategy than I can here in the humid, acid rain filled northeast. And the methods I have good luck with on a summer only vehicle can be very temporary if used on a car driven through winter snow and salt. And none of it may be effective for someone living in India that has to deal with monsoon rains and humidity that corrodes everything.
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