At what point is it not worth it to fix rust on a classic car?

Question:

My 1968 Mustang notchback coupe is in good mechanical shape, its interior is excellent, and it has only 76,000 miles on the odometer...but it has some rust through in the door corners, floors and sub-frame. Is the car savable or should I consider selling it for parts?

Answer:

“Rust”...the very word sends chills up 4 every old car lover's spine. It can cost thousands to fix, and can render a mechanically sound car unsafe for the road, and even un-repairable. Obviously, the best way to avoid having to deal with rust is to avoid rusty cars. But it sounds like you are in love with this car, and love conquers all—much of the time. Without seeing the car it would be difficult to tell you what is involved in your particular situation.

A good body shop can give you a rundown of the problems, or you can do a careful assessment yourself and decide whether you want to try to save the car, and whether you want to tackle the problems yourself. One important thing you have going for you is that everything is available for your car including floors and body panels.

Start by checking the body, underside and sub-frame carefully for rot, rust and poorly done repairs. Cheap fixes with plastic filler have no structural integrity and will actually attract moisture. Clumsy, lap-welded repairs will just rust away that much faster too, because welding carbonizes the metal and causes it to rust in areas that can’t be reached.

Get under your prospective project and poke around with a screwdriver. If the metal is soft and spongy, it will need to be replaced. Use a small magnet and make sure it sticks to lower sills, fenders, doors, floor pans and the rear skirt under the trunk lid. Don’t use a powerful magnet for this, because a big one could actually stick to the car despite extensive paint and plastic.

There are several good strategies for ridding your car of rust, converting it or at least arresting its progress.

The best thing to do is to sand off every last particle of rust or cut it out and replace it with healthy metal. But if—as is the case with many of us—you can’t afford to go to that much trouble, there are still some good techniques that will stop the progress of corrosion in certain cases and help make your vehicle last for years.

The first line of defense against rust is to clean the underside of your car and its fenders of dirt. Soil soaks up moisture and salts and holds them against your car’s precious tin where they will wreak havoc. A pressure cleaner can be used to get dirt off, but don’t shoot water inside the engine room. You could damage irreplaceable electrical components if you do. Even when your car is properly restored and rust-free, it is a good idea to wash the underside of it and dry it thoroughly with compressed air after a tour.

Each sheet metal panel of a unit-bodied car like your Mustang helps support the others, so if any of them are unhealthy, the whole car can become unsafe. If floor pans and B pillars are rusted out they will allow the car to twist and flex, causing metal fatigue and further problems. If the rot is advanced, a collision in such a car could be deadly.

Patching rusted sheet metal takes practice, skill and a MIG (Metal Inert Gas) or other welder to do the job. The only alternative is to find a pro panel beater to do the work.

Panels must be formed out of metal that is the same gauge (thickness) as the metal they replace, and they must be made of steel with the same carbon content. (In the case of structural members you may want to go to slightly thicker metal.) If you hit the new metal with a pass from a surface grinder and it gives off sparks of the same color as the old metal, the carbon content will be about right. You also can get healthy tin from a donor car to save money.

Make patterns from stiff card stock, and make sure they will fit before cutting up sheet metal. You can use body hammers and a shot bag (or a canvas bag filled with sand) to shape compound curves. Butt welds are best for thin metal floor pans. However, some of the heavy, structural pieces under a unit-bodied car are lap-welded in as doublers for extra strength. In that case, coat the inside of the new metal with cold galvanizing compound before welding it in. Cold galvanizing compound will not affect the weld and will help protect the metal.

Nothing will make rust into healthy metal again, but you can convert surface rust to a benign substance called magnetite that does not corrode. I like POR 15 for this. It is especially good for hard-to-get-at places such as inside rear quarter panels and reinforcing structures. You only need to wire-brush the loose corrosion off, then make sure the panel is free of grease and oil before applying it. You can simply brush it on in areas that won't show.

There are places where the above solutions prove impossible or impractical, such as the insides of frame rails and other areas that you cannot reach between the frame and the body of a car. In those cases, cavity wax is the answer. It forms a film over the rust that keeps moisture out and retards further corrosion. It isn’t effective for areas that are directly exposed to the weather, but it is great for those difficult-to reach, sheltered cavities. Use a sprayer with a long wand to shoot this stuff in.

Finally, strip out and re-caulk all seams in your car's structural metal. Caulk gets old, brittle and stiff from environmental contaminants and won't do its job.

Also, unless you are going for a factory-fresh appearance on a car that you will be driving only in fair weather, you might consider undercoating the chassis. Don’t do the engine, transmission, driveshaft or any of the steering components or grease fittings, but be sure to cover all the sheet metal.

These strategies, plus proper storage and maintenance, will keep your car in good condition for years to come. Dont forget to hose off the underside to remove dirt and salt and be sure to store your car indoors on a concrete or wood floor and not on dirt as in a barn, because soil sucks up moisture that will condense on your car’s chassis and rot it.