Something that just occurred to me: The cross tube where the centrestand is attached is NOT the lowest part of the frame.
You will note that the damage to WF's frame is worse on the left side than on the right. I would expect that this is probably typical because that side is lower when the bike is on the sidestand. This would also contribute to a higher incidence of rust in the (removable) left lower frame tube than the right lower tube (the open ends of the removable tube don't help either).
I'm sure, however, that it is not always the case. If a bike was stored on its centrestand for an extended period or attached to a sidecar, any moisture (condensation, road spray - salted if used in winter, rain water)(ever use a pressure washer? not recommended for bikes...) that gets into the frame will probably end up inside the right lower tube.
In any case, rust on the bottom of the frame is definitely something that anyone buying an old bike should look for.
This doesn't look too bad, does it? How far is it from cracking like the other side? I wonder how that sort of damage to the right lower tube would effect the handling? Or how long it would last if a sidecar was attached to it?
1) We should all examine our frames periodically for this sort of damage.
2) It might be a good idea to put a good quality high creep rust preventative oil in the frame of any bike to help stop the growth of rust where you can't see it. On RustCheck's
recommendation, I have been using their Rust Inhibitor (red label) inside the frame of my winter machine. I think I will start putting it inside the 'Wing's frame too.
BTW: I experimented with this stuff before I started using it. I sprayed approx. 3/4" sized spots of a number of spray can oils/greases onto an 18" long piece of very rusty angle iron, a couple of inches apart. A day later I couldn't find the spots from WD40 and silicone spray and the white grease had not changed, but the RustCheck red had spread about 2" on each side. Over the next few days it spread over most of the piece of angle iron and it eventually became so thin I could no longer see a distinct line where it ended. And all that from just a few drops. A few ounces inside a frame, left to creep for a couple of weeks, should form decent layer.
Also, someone who cuts up dead cars once told me that when they cut through the door of a car with any other oil treatment the area inside the folded seams is dry, but on cars that have the RustCheck decal the oil has spread through the entire seam.