Join Date: Dec 2004
Bikes: 92 Giant Sedona ATX Custom
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Grit inside is not always rust, in fact critical rust is scales built up on the walls. The places to check for rust will be the downtube-into bottom bracket shell and chainstays. These areas tend to concentrate what water enters the frame. If you can get a view in with a small mirror or probe, look for scales or orange rust(there are colors). That is the more active variation.
If the surface is not scaled but still has rust(even if it does not) use a product called Extend. It applies like a paint, through a wd-40 style nozzle spraycan. Once applied it will convert the rust into an inactive surface coating and it will not be a problem later(if coated adequately).
If there is scale buildup(you can see edges or major pits), ask yourself what service duty/life you are asking for. If it is not on welds and the pits are not major, then it can be used, but decidedly limited in lifespan, as these points can cause stress or continued rust issues.