Here's a thread where I did some extensive fabrication to save a 55 wagon tailgate (essentially a door)
http://www.trifive.com/forums/showthread.php?t=53428 Where the outer skin had only a few pinholes showing, once opened up it exposed how extensive the adjacent deep pitting was, and that more "breakthrough" was inevitable. Shown here with only three pin holes to the outside, and much more lurking:
http://i5.photobucket.com/albums/y1...20Restoration%20Album%203/Picture047.jpg Of course not every situation dictates removing the skin altogether where you would see the extent of such damage. But given the damage we could see, until something like that is exposed and fixed, it is likely that additional rust will follow shortly. I don't think an application of POR over top of issues as those in the above picture will prevent further rust, IMO many of those pits are at a point they would continue to degrade even under the POR. For those iffy areas where you haven't found rust through but suspect damage, I normally check the structural integrity with an ice pick. If the metal will hold the ice pick from going through, it will hold paint. If not, then the pit would have rusted through with a couple years regardless, right through that new paint job.
To help identify lurking pits from the outside, if you sand the panel to bare steel and notice small, darker circles about 1/16 or so in diameter, this is a good spot to check with the ice pick. More often than not, it will go right through the weakened area. But this is the time to find those areas, before the paint goes on.
Looking at the same area of pinholes, notice the dark circles on the outside in the immediate area, then compare the degradation of that same area on the inside.
http://i5.photobucket.com/albums/y1...20Restoration%20Album%202/Picture545.jpg http://i5.photobucket.com/albums/y1...20Restoration%20Album%203/Picture047.jpg For future rust prevention in these areas, when installing new skins, my preferred method, as shown in the tailgate thread, is to abrade the inner door and inside of new door skin, then epoxy prime. Once the parts have cured the "mating surfaces" are scuffed and parts assembled/spot welded as needed. Then I'll mix some fresh epoxy and pour inside the door, rotating the door so the paint seals around the perimeter of the seam (as required)
For areas that have not been opened up for skin replacement/rust repair, I'd suggest looking for those dark circles that indicate pits are coming through, and use the ice pick as needed to test structural viability.
