Using a straight edge, and holding 6 of my planes up against sunlight,
the gaps were bad.

I sanded and sanded, going as low as 60 wt sandpaper. Then 120 wet dry.
After two weeks of this, the soles still had hollow patches near the
mouth and heel & toe dips. The guys on oldtools.org informed me that
only my No. 3,4,4 1/2 and 5 plane need to be dead flat. I was getting
nowhere, and had expended $25 on sandpaper.

So I asked my neighbor, a machinist by trade, if his workplace had a
good surface grinder. He said they make MRI equipment for hospitals. I
gave him about 50lbs of planes and he returned two days later with
transformed tools.

The soles were within .0005" flat, with no hollows anywhere. And, the
soles are at 90 square with the sides. I gave him $50 for his trouble
and consider myself lucky. These are all Pre-WWII planes. Some are pre
WW-1 planes.

I've heard of guys using a belt sander clamped in a vise to accomplish
the same thing. By hand, you might be digging the proverbial tunnel to
China.

Gary Curtis
Los Angeles