Re: Polyurethaning log wall
[QUOTE=Jimandamy2;8756] I suspect I may need to thin out the final coat or warm it up to about 80 to thin it more.[/QUOTE]
The more thinner you put in it, the more apt it is to run. Warmer would be better (to a point). You didn't say what the temp was overnight......or whether this was oil-based or water-borne. (I hope you didn't spend the night inside the house where you did this cause the offgassing would choke a horse if there were many sq. ft. involved.)
Varnishes in general are prone to running on vertical surfaces as they don't flash off quickly like lacquer and shellac do. You certainly aren't the first person to have this problem and you won't be the last. <G>
If the wood was bare when you started, the wood sucked up much of the first and some of the second......most likely. The third coat wouldn't likely be absorbed at all and so the tendency to run would be much greater. The problem might have been exaggerated by a poor quality brush, by applying the third coat before the second was dry/set-up or simply by applying a little too thick of a coat the third time around. Thinner applications/coats are better (not meaning thinned material). Too low of a temp would also contribute to this tendency becasue the longer it takes for the material to dry.....the more time it has gather and run.
I suspect the runs are now too stiff to slice off with a single-edge razor blade, but may also be too soft yet to sand without clogging your paper or having the material crumb like a pencil eraser. You may have to wait a few days or more and then sand the runs off with some 120-grit or finer. I'm afraid there is no such thing with WB or OB poly as patching in and getting a totally invisible repair. You'd end up with a detectable halo around the perimeter of the repair/recoated areas. You'll have to recoat the entire surface to get the best result.