: . . . I have a few runs in the varnish (after 3 coats), and thus
: the question "What should i do about them?" Do i scrub away with
: the 400 grit, or with something more coarse? Do i run a risk of damage
: with something too coares or what?

You can certainly remove those runs with 400 grit sandpaper. It will just take a lot of time, and you'll wear out or fill in a lot of sandpaper. Go with something considerably coarser. 100 or 150 grit should do, depending on how aggressively you go after the runs. Yes, this will leave some slight scratches on the surrounding varnish, but those should fill in completely when you apply the varnish. In the event thre are still some valleys, a light sanding with 200 or 300 grit -- or even your 400 grit -- sandpaper should level things off.

:...Also, am i ok to use a foam
: brush for the final coat (i'm looking for the smoothest finish possible),
: or should i use a brush made for varnishing?

Check the archives. I believe Spidey was the one who posted a rather detailed explanation of how to use a rag to wipe on a very thin layer of varnish. This seems to have been tried by a few others with very good results.

Hope this helps.

PGJ