Max:

I would probably do more than just sand. But then I have to put out a
warrantable product so my take may be different than some. Solvent
based finishes often do not lend themselves willingly to adhesion of
other products, especially after they have aged, and worse still if
they are damaged.

Before anyone craps a banana, this of course excludes all build
finishes. But in this case, we are talking varnish.

As a sidebar, if the clear finish (no matter what it is) is dried out,
cracked, discolored, crumbly, or the wood underneath is discolored, the
finish is gone. All you are seeing when the surface is in this
condition is the last 5% of the finish, just the part that hasn't
fallen off. In this case, simply sanding off the crumbles won't work,
you must strip and sand to get to a good bondable substrate.

Anyway, it begs the question Max, how do you know it's varnish? When
we refinish a door, we have found that most homeowners and many
contractors use polyurethane for original finish, and then for
refinish. If it is a homeowner job you are looking at, you can almost
bet it was poly. Varnish and poly are two different animals, and if it
is poly, you should count on stripping the door before painting.

Me, I would strip anyway to be absolutely sure, especially since you
are probably talking about one side only. The oils, resins, and
hardners in the varnish (if that is what it is) have no doubt
penetrated the wood over the years, so there are things in the wood
that could kill your adhesion. And since you are going back over it
with paint, you wouldn't have to have the surface "perfect" before
painting, especially if you go back with oil based paint.

If you want to go latex, strip, coat the stripped side with KILZ 2 to
seal the resins in the wood and to assist in bonding, sand lightly
when dry, vacuum up the particles left from sanding, then put a couple
of coats of paint on it.

If you don't want to strip, clean the door up, sand the snot out of it,
vacuum up the particles, wipe down with thinner, seal it with the
original KILZ (for bonding purposes) and have your paint store match a
quart of oil based urethane to your latex color. It may be cheaper
(stripper for latex = $20; quart of custom mix oil base = $12) and
easier to avoid stripping and just to bite the bullet and buy a quart
of oil based paint. A couple of coats and you are finished.

Hope this helps.

Robert