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Discussion Starter #1
How can I distinguish between "surface rust" that can be sanded away from what I'll call "rust damage" that needs to be cut out and replaced with new sheet metal?

I ask because I have heard of a Mustang that has "some surface rust" that is for sale at a reasonable price if the owner's verbal description turns out to be true. I have not yet seen the car. The car is not posted on any auction site, so no pictures are available. The car is nearby so I may check it out tomorrow.

I know I can check places that commonly have rust problems to see if they are badly rusted. The "surface rust" is probably just hiding something if those are bad. However, I am not sure what to check if those look OK. Is the body usually OK if the places with common rust problems look good?
 

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If you can remove the rust down to clean metal in a 6" square area by hand with a piece of sandpaper in one minute, it's surface rust.
 

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Discussion Starter #3
Is there any other way to tell? I don't think the seller will let me sand his car as a test.
 

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Does it look like that will do?
 

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Discussion Starter #5
I have not seen it yet. I am new at this, so I am reluctant to trust my eyes. I realize there may not be a better way to tell the difference between surface rust and rust damage, so I thought I would ask the experts.
 

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If it's flaking off, it's not surface.
 

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You've probably seen cars in junk yards or that have been sitting outside for years. The paint gets thin and rust sets in. This typically happens on the weather-beaten sides of the body panels. It has a distinct look to it. Typically there's no hard edge where the rust just stops and the paint begins. As has been said, if the paint appears thick and is flaking surrounding the rust, then it probably came through from the back side of the panel. Exceptions might be caused tho if it was a Maaco or other budget paint job that didn't adhere well, but the flakes then would typically break off in shear lines and then the rust starts afterwards. Rust through typically starts as a bubble under the paint and once the blister is broken, leaves rather jagged lines where the paint has broken. Hope that helps.
 

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I had an area on the roof of my Fastback like this. I took a sanding pad on my buffer with 40 grit, knocked it down then went back over with 80 on a DA. Any deep pits were ground out using a die grinder. I then epoxy coated and filler in small areas that were ground out.
 

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I'll show you two floor panels so you can see the difference.

Surface Rust:
seattle67 :: P8044383.jpg picture by lemonshindig - Photobucket

No pitting or holes. Just orange color on top of the base color. The metal is still strong and has little to no give in it. All apparent rust can be cleaned up with a wire brush or sandpaper in a matter of minutes, and then the metal can be repainted and the repair is done.

Rust Damage:
seattle67 :: P8044366.jpg picture by lemonshindig - Photobucket

Holes and pitting. Pitting are little divits in the metal, kind of like potholes, where the rust has started to eat through the metal but hasn't made a complete hole yet. Pushing on the metal provides little resistance, an indication that the rust has thinned out the panel. Repairing this requires cutting out the damaged area and welding in a replacement panel.

Good luck
-Matt
 

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Excellent advice above. Truth is very hard to tell. This is what I do, first I tell the seller what I'm about to do, seller is the one that represents it as surface only rust. Find the worst visible spot as described above, take your trunk key and use the small end and push it into worst spot, if it gives or actually pokes thru then you have your best indication of serious rust (I'm talking body panels and floor boards here). If the seller won’t let you try this then they know it is not surface rust.
For a Mustang the best advice is to know where to look, floor boards, frame rails, etc.
For heavier steel (frame rails) inter-granular corrosion (vise surface) can have little visible indication but can be a huge problem, this is where you want to sand. If corrosion gets larger as you sand then you have a potential problem.
Spend a lot of time under the car !
 

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I'll show you two floor panels so you can see the difference.

Surface Rust:
seattle67 :: P8044383.jpg picture by lemonshindig - Photobucket

No pitting or holes. Just orange color on top of the base color. The metal is still strong and has little to no give in it. All apparent rust can be cleaned up with a wire brush or sandpaper in a matter of minutes, and then the metal can be repainted and the repair is done.

Rust Damage:
seattle67 :: P8044366.jpg picture by lemonshindig - Photobucket

Holes and pitting. Pitting are little divits in the metal, kind of like potholes, where the rust has started to eat through the metal but hasn't made a complete hole yet. Pushing on the metal provides little resistance, an indication that the rust has thinned out the panel. Repairing this requires cutting out the damaged area and welding in a replacement panel.

Good luck
-Matt
Most areas that rust through the metal are located in areas where moisture collects and doesn't evaporate. These areas are mostly on the lower parts of the car like on the backs of fenders. bottoms of the doors or bottoms of the quarter panels. When we look at a car prior to making repairs we will lightly hit these areas with a body hammer to see if the rust indicators seen on the surface have weakened the metal.
 

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Discussion Starter #15
Thanks again for the fantastic advice. If I am crazy enough to consider buying another Mustang I will definately post some pics.
 
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