John,

If these doors are very old, there is a chance that there is metal such
as hidden nails in the wood that you want to remove. This is the reason
why those walnut trees in your backyard have no market value as lumber.
No mill wants to risk destroying an 8' or 16' planing blade on somebody's
old clothesline nail buried under the bark. Likewise, I don't want to risk
ruining my cutting tools helping somebody cut some lumber which may
have a nail in it.

This is how I would attack it using a power plane, if I was positive there
were no nails:
- Draw a pencil line on both sides of the door where the cut should be.
- Clamp straight scrap lumber on both sides, lined up with the
pencil lines. Ideally, the total width of the door and the two other
pieces of wood is greater than the width of the sole plate on
the planer.
- Now use the power plane. The scrap lumber provides a great guide to
avoid cupping the wood or cutting too deep. When the edge of the
door is even with the scrap lumber, you're done with that door.

I bought an inexpensive power plane for a friend as a gift. He loves it.
He milled his own cedar bookshelf lumber from discarded material and
then used the power plane to dress the edges. The plane cost $19.99
from Homier (Roaming truck sales and Internet orders). I used the cheap
one a bit and it works just as well as the one that I bought many years
ago for $100. The bearings in the cheap model may only last for a few
hundred hours of use, but that is plenty for the average homeowner.

I suggest finding an inexpensive power planer. You'll easily get your
money's worth on this project and others. It is a tool that any serious
do-it-yourself type should own. Buy a spare set of blades (about $4-5)
just in case you hit a nail. I've never dinged my blades, but if I do then
I'll keep that set for planing questionable lumber.

Obviously, experiment with the power planer before attacking the doors.
The depth of cut is adjustable from moderately deep bites down to
paper thin shavings. Also, remember that the blade is going to coast
and continue spinning for a bit after you release the trigger. Watch
your fingers.

By the way, don't forget to seal the fresh cuts on the door with a quick
coat of paint. If the cuts won't show (top or bottom of door), then a
coat of shellac is even quicker - it just takes minutes to dry to a light
amber color.

Good luck,
Gideon