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I could probably have been president if I had given this important
technology to the Chinese.

To fill areas with micro, like any other area you want a good bond on,
clean it and scuff it. Mix some epoxy and paint it on the area you are
going to fill. Heat the epoxy you just spread with your heat gun till it gets
thin and with a plastic bondo spreader squeegee up as much as you
can and putit back in the cup,  mix more epoxy and mix micro balloons
in to it 'til it is a dry mixture, so dry that it almost won't stick to the side
of the cup. Now with a putty knife glom it onto the area you
want to fill.

Notice how it wants to stick to the putty knife when you try to spread it?
Don't try to spread it, just pile it on the area. Now hold your heat gun in
your left hand (you are right handed aren't you) and a shiny steel putty knife,
about 2 "wider than the area you are going to fill and heat it (hot) with
the heat gun and spread the micro. Don't be artistic, and leave it high
(it is easier to sand down than sand, fill and sand again.
Also fill well outside the area you think you want filled.  When the
least bit of micro sticks to the blade stop(or keep the blade hot enough so
that none sticks), clean the blade and heat again.  The hot blade moves
the micro easily and doesn't cause the micro to pull back up. Under no
condition heat the micro with the heat gun, if you do it will stick to anything
including the hot blade. This method gives you a well bonded fill with
many less pinholes.

On sanding micro - let it sit as long as possible - it keeps shrinking long
after it is hard. Use 80 grit automotive (yellow) sand paper
on a body
plane. Don't try to save money on sandpaper. The automotive  paper
doesn't seem to wear out, but the abrasive gets dull. The 80 grit will slide
right over the prepreg if you don't push down. The sandpaper cuts
better if you don't push down.

Bill N7WB