...is like painting with mucus.
On the recommendation of a professional painter, my wife and I spent two days this week applying spar varnish to the ceiling of our front porch and the soffit around it. While the end result looks lovely, the actual overhead application of the varnish (using a pair of Shur Line 9" pads and a couple of extension poles) was horrible. Imagine having snot dribbled all over you and everything around you. Clean-up (using paint thinner) is more time-consuming than cleaning up oil-based paint because the varnish is so thick. It could have been worse, I suppose: we could have tried to do it with paintbrushes.
Updated: two tips. The first is about thinning the spar varnish: yes, you can thin it, despite what the label says -- about a teaspoon of paint thinner in a quart of varnish turned it from the consistency of cold honey to something more like heavy cream. The second is to roll it on with a 1/4" roller then back-brush it to get the right finish. For our project, which already had two coats of spar varnish, these two tricks combined to enable us to quickly get a mirror-smooth finish with no visible imperfections.