General Underseal chisel
It's often useful when spotting a small suspect area of delaminating (original) VW underseal, to investigate further. Invariably there'll be damp and water behind it e.g. dark rusty, rather than lighter coloured dry rusty metal (ginger!).
A few minutes shaping an old wood chisel on a bench grinder or surface stone can produce a good tool for easily cutting/shearing the whole thickness of underseal away, and picking apart any areas holding damp.
I used a reasonable Draper 3/8" chisel, and occasionally re-shape and re-sharpen it. Rough and ready first, then a smooth stone to re-sharpen. A 1/2" or larger wood chisel might be useful too, but requiring a lot more force or moderate blows from a club hammer o edge it along under the old sealer.
The idea when shaping it, is to severely round off the corners to stop it digging in, yet retain some sharpness around those edges. Long slivers of u/seal can then be sheared off right back to the original paint, without too much damage to the metal surface itself.
After revealing a rust patch or pocket
The aim is to get air inside the rust pocket, dry it out, de-scale and then treat (at least temporarily), say with an anti-rust paint or quick soaking inside and out with a a-r spray (Dinitrol RC900 is my current favourite)
If that process produces holes then so be it; work it back to good metal if possible, leave to dry a day or so (hot air gun in winter) and then treat everywhere. Seal up temporarily with a thin plate (ally?) stuck on with RTV or seam sealer...
Add it to your list of places to sort permanently :)