A year or so back, there was a tip (in FWW) by Jimmy Carter on
sharpening jointer blades in-situ. I did not pay much attention at the
time, as I did not own a jointer, I have now acquired an old one with
3 sets of blades, 1 set is OK, just, but the other 2 are worse for
wear.

What was his technique?

Barry Lennox