Here's a quote from Paul Sellers' blog:
Firstly, and I have already said this elsewhere, plane irons need not be dead flat at all.
Secondly, they need not be replaced with any other make of iron and certainly not thicker ones.
Thirdly, if they are bellied they need not be abraded to flatness.
Fourthly, if they are hollow they are ready to go and need minimal restorative work beyond minor abrading and polishing out along the back of the cutting edge.
There, I have just saved you an hour or two’s work.
Could someone explain his point here? If the back of the blade isn't flat, how can it be polished completely across its bottom edge?