Dry Overspray

I would sand with 1000 grit wet sand paper.  Put a bit of soap in a bowl add water and sand paper.  Sand it in a corner not noticeable wipe off with an old tee shirt. 

I smooth out Poly that way all the time.  Not sure it will work with varnish.

Mike

If I were to try to fix this with what I have on hand and the methods I know it would be...0000 steel wool. If that is too flat follow up by rubbing with pumice and oil. Rottenstone and oil after the pumice if you want  more sheen.

Toothpaste? I know they use it to polish violin varnish.
https://www.scavm.com/Toothpaste.htm

Added later 16 min 19 s:

I don't know about wood and varnish, but with cars to achieve a "show car" paint job, it is a progression of sanding to remove any orange peel and imperfections. This starts with a grit sufficient to level any orange peel, and then progressively moving to finer grits, until maybe 3000 and then moving to polishing compounds. I suppose one could just keep progressing to compounds with increasingly finer abrasives, but there comes a point the human eye cannot tell the difference.

As for a $1000 paint job on a car and a $10,000 show-car paint job, there isn't really a huge difference in the actual painting. The difference is the amount of prep BEFORE painting and amount of elbow grease expended AFTER painting.

Unless the surface is leveled, it seems that any polishing is only going to produce a bumpy surface that shines.

Added later 06 min 33 s:

Ah, just noticed the title says, "over-spray."  In that case, then leveling probably isn't needed. Just some kind of polishing compound abrasive enough to remove the over-spray. Since I'm not a woodworker and don't have any fancy wood polishing stuff on hand, I'd probably use toothpaste.  But then, I have dentures and don't have any of that around, either. Maybe the container of Barkeeper's Friend from under my kitchen counter?