glassartist wrote:
I am refinishing 80 year old fir floors. There are some bruises that are
quite pronounced and round (probably from furniture feet, like beds). I've
done a lot of reading and I've tried things I've thought of, but so far
they resist most of my efforts.

Read: sand 45° with coarse grit rotating 90° until gone then finish with
grain.

Result: I seem to be using a lot of good floor to get rid of blemishes. It
seems it would be better to address each blemish on its own or I'll loose
a lot of my boards' depth.

That is the unfortunate fact.

My thoughts: using heat gun remove most of the shellac/varnish and then
it's easier to hand sand, but it makes a hollow.

Whether the hollow or blemish would be worse depends on the situation.

Question: Can I fill the hollows with a "filler"?

Possibly, probably not. Most of the times the blob of filler looks
almost as bad as the blemish. You can use artist brushes and apply
stain to the filler to replicate the wood grain. Easier said than
done.

Can I mix a filler using
my sanding sawdust? Is there a commercial product that will finish like
the rest of the floor after I stain it all?

Sand the floors most of the way done. The blemishes and divots will
stand out as you'll see the remaining floor finish and the flatter
areas will be sanded bare. Hand scrape the remaining finish off a low
area. Wet a rag, wring out a bit, lay it on the low spot you want to
raise. Do not saturate the wood. Try not to go outside of the low
spot with the water/rag as that will create other problems. Move a hot
iron around on the wet towel. This will raise the wood. It works most
of the time, but not all of the time. It works best on more or less
uniform depressions. Let the wood dry for a couple of days at least.
If you're in a rush, hit the treated areas with a heat gun or hair
dryer (longer blowing time is better than hotter temperature). If you
have a moisture meter, so much the better. Finish sanding the floor.

In general a refinishing eats up somewhere between 1/16" and 1/8". All
of these decisions come down to cost. Do the best thing for the floor
that you can afford and I'm sure it'll be the right thing to do.

R