C2 Frame surface rust

[Oliver S.]

As far as I assume frame rust starts always from the inside and penetrates to the outside, isn't it?
If you discover that you have a bit more surface rust somewhere on the outside (however the frame is still solid) and remove it there is some minor recession compared to the other area around. This may only be a visual issue but how is this treated? Just paint it over?

Oliver

[Chuck S.]

Re: C2 Frame surface rust

As far as I assume frame rust starts always from the inside and penetrates to the outside, isn't it?

No...two types of rust are being discussed here.

Surface rust typically means an early onset corrosion that is progressing slowly, and which consequently has done very little structural damage. It could be on the outside "surface", OR on the inside "surface" where it can't be seen. Moisture and minerals tend to collect in certain areas inside the frame and do extreme damage without you knowing. If rust has progressed from the inside out...your frame is usually toast, or will require major repairs to be safe. You will first see "inside rust" on the outside as a small rust pimple, or maybe a small rusty hole or "crack"...that is a first arrival "pit" that has penetrated through the full thickness of the frame material from the inside.

...If you discover that you have a bit more surface rust somewhere on the outside (however the frame is still solid) and remove it there is some minor recession compared to the other area around. This may only be a visual issue but how is this treated? Just paint it over?...

Under continued exposure to a lot of moisture and salt, surface rust progresses to "general pitting corrosion" (We've seen this type of corrosion here recently on some half shafts. ). In exteme cases, general pitting corrosion may progress to "scaling" where your former frame (now rust) is delaminating into small "sheets" of ferrous oxide.

By contrast, surface rust can normally be easily removed by media blasting (preferable), or thoroughly cleaned mechanically in very light cases, and then "repaired" to undetectable by simply priming and painting the frame. Simply "painting over" surface rust is a very bad idea...unless you use one of the "anti-rust" paints, air passes through the paint coating and continues to feed the corrosion process; all that painting over rust does is give you a false sense of security.

General pitting corrosion will have a material impact on the strength of the frame, and may be cause for frame retirement in extreme cases. Rust (ferrous oxide) always has much more "volume" than sound steel; however, when general pitting corrosion is removed, it ALWAYS leaves a void, with the extent and depth of the "recession" dependent on time and conditions. If, after the pitting corrosion is removed to white metal the safety of the frame is not compromised (a judgement call with experience), you can "skim" the pitted surface with bondo, prime and paint...this assumes you don't plan to "campaign" the car on road or track.

[Joe L.]

Re: C2 Frame surface rust

As far as I assume frame rust starts always from the inside and penetrates to the outside, isn't it?
If you discover that you have a bit more surface rust somewhere on the outside (however the frame is still solid) and remove it there is some minor recession compared to the other area around. This may only be a visual issue but how is this treated? Just paint it over?

Oliver

Oliver----

If you have any questions regarding the possible structural integrity of a frame as a result of internal corrosion, I suggest probing the frame using an ice pick or sharp awl. Probe in the areas where internal corrosion most often shows up. Apply significant force to the ice pick when you're probing----you won't hurt a sound frame in any way doing this. However, if the ice pick "goes through", you haven't damaged anything worth saving. You'll need to probe rather extensively to ensure that there are not any "weak areas".

This method will not tell you that there is no significant internal corrosion, though. It will just tell you that you have or don't have major internal corrosion that constitutes an immediate safety hazard with continued operation of the car.