Having given the issue some more thought, I think it might be the fact that the primary bevel on the iron is close to 45 degrees. With the bedding angle also at 45 degrees, putting a secondary on it meant that the cutting edge of the iron was actually higher (when viewed from the side) than the back of the secondary bevel.
I've borrowed the scrub from a friend who inherited his grandfather's tools, but doesn't yet use them (I'm trying to change that!) so I'm obviously reluctant to mess around with it too much. All I've done to it is to de-rust the blade and lightly rub the (wooden) body with wire wool. Hence I've stayed away from major grinding. The blade is lovely, tapered from top to bottom, and at least 3mm thick at the business end, it's stamped G BALDAUF. So I guess the queston is, is what _should_ the grinding angle be on this tool? I have to say, once I removed the secondary and returned it to how it was before I sharpened it the plane started to cut beatifully again.
