The MGA With An Attitude

UC-201 presents the basics of UNDERCOATING and RUSTPROOFING function and application.

At 01:11 PM 2/23/02 -0600, Jim Kacena wrote:
>"I'm in the process of replacing my brake and fuel lines. As I proceed, I'm sanding the rust under the car off. Any recommendations of a product I can put over the remaining rust so it would adhere to the remaining rust, like a black Zebart product????? Something easy that I can put on like a paint brush??"

As strange as this may sound, undercoating has two primary functions, sound deadening and abrasion resistance, but it is never used or intended to function as the primary rust prevention coating. If you apply any undercoating product over rust, the coating will soon fail and the rust will continue hidden from view until the metal is gone. All it takes is one tiny pinhole or a crack in the coating to allow moisture through, and all of your best efforts will have gone to waste. Such undercoating can easily develop a crack in a short time through vibration or temperature cycling. In some cases moisture can even penetrate through the undercoating by osmosis. There are a few ways to stop rust, but hiding it under a layer of undercoating is not one of them.

1.) The very best way to banish rust is to remove all of it, usually through abrasive process such as sanding or sand blasting, until there is only clean shiny base metal remaining, and then prime and repaint it with weather resistant paint. Ordinary body painting processes are sufficient for this, as long as the area is not subject to abrasion from road materials.

2.) The second case is when most of the rust has been removed but small traces or rust will be left in place, such as power sanding to the bare surface but leaving small amounts of rust in pit mark depressions. To prevent this rust from spreading it must be completely and permanently sealed away from any future contact with the atmosphere. For this there are paint-on products such as POR-15 (not the only product line). Check here:   This material has the appearance of a hard epoxy paint, with the special feature that it dries to a very strong and tough finish but remains slightly flexible. As such it can seal the surface, and it will resist cracking by means of its ability to move with the parent metal during vibration and thermal expansion and will even tolerate mild abrasion. Some of these coatings are ultraviolet sensitive and will fade or discolor with exposure to sunlight, so in areas where appearance is important they should be finish coated with regular body paint. NOTE: This method does not work if there is perforation of the sheet metal. The tiniest little pin hole through the panel will allow corrosion to continue from the opposite side of the panel. In the case of perforation both sides of the panel must be treated in similar fashion to affect a complete seal.

3.) The third and worse case is where the surface is badly rusted with blisters and/or flakes of rust in great prominence. As always it is desirable to first remove as much of the rust and damaged material as possible. In some cases where access is difficult, such as inside of a door or in tight corners, it may be impossible to remove some of the rust. Heavy rust is like a sponge, looks solid but has a lot of empty space inside. It will at first take a layer of paint on the outside, but will remain free to breath inside, and it can easily retain moisture inside where it can become trapped under the new coat of paint. In this case the paint doesn't have to be cracked, as the moisture is already there under the paint, and the rust will continue until the paint does blister and crack. And it is extremely difficult to fill or empty or to seal off this space within the rust itself.

To treat the surface and the rust in this case there are rust neutralizing products such as Loctite Extend (not the only product). Check here for product in formation:
This material actually requires the presence of rust for it to work, and will be of very little benefit on unrusted surfaces. It soaks into the rust and attacks it, chemically changing the very volatile red iron oxide (FeO2) to a very inert black iron oxide (FeO3). Black Oxide is in fact a very stable rust preventative surface treatment often used for steel parts in lieu of electroplating, and is usually created in an electrolytic process. You may know of some hand tools that are black matte finish and do not go rusty. That is black oxide. The new steel MotoLita steering wheel in my MGA is also black oxide coated. See here:

The brush-on treatment requires application in two coats, 15 to 30 minutes apart. When applied it will immediately (within 30 seconds) begin to turn from milky white to dark violet or black as it goes to work. Allow to air dry only 15-30 minutes and apply a second coat. The rest of the instructions can be found in the product data sheet noted above. This is only the rust treatment, not a finish coat. What it does is to eliminate the rust by converting it to black oxide, which you can paint over. Allow to dry for at least 24 hours, and then paint over it with your favorite body finishing primer and top coat. I personally treated the inside of the (very rusty) doors on my MGA this way during restoration in 1986. Today the car has another 170,000 miles driven in all kinds of weather, and the paint is still there with no rust. And it is not for lack of moisture, because the doors do get wet inside occasionally (fairly often actually).

So you can remove all rust before painting, or you can remove most of the rust and attempt to create a perfect seal over the small amount that may be left, or you can neutralize the rust before painting over it. But in no circumstance could you simply paint over rust and ever expect the paint to protect the surface underneath. Any active rust left underneath the paint will continue to propagate, causing paint blistering and cracking and more rust. The products noted above and other similar products are available through your local automotive paint supply store. Just walk in and ask, and you will usually get lots of free advice, a little more education, and your choice of even more products to suit your every need.

Hope this helps,
Barney Gaylord

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