Hippie Guy, I beg to differ.
Most finishes are not impermeable to moisture, and satin finishes are often particularly thin, thus allowing for moisture...and sweat...to migrate through to the wood where there may very well be a grain raising effect. The least permeable finish is probably gloss polyester, especially with an under-sealer of epoxy, the most permeable would be satin lacquer or urethane, and French polished shellac.
I once worked for a company that was making Victorian-style refrigerators...the company was called "Ice-a-Boxa" which is Italian for refrigerator...according to Pat Kuleto who owned the company. One of my jobs was to find the most moisture resistant finish possible for the walnut and oak we used to build the exterior cases. It wasn't easy at that time...1983 or so...and I learned a lot about what did and didn't work for moisture resistance, state of the art, that time period.
The armrest is a good solution, as is making sure you wipe the instrument off with a towel after playing it. Ultimately, an instrument played is an instrument that will show wear, and to me, that's part of the beauty of older instruments...you can see the evidence that they were used and loved.
Note too that sweat is not just water, and differences in body chemistry can make a huge difference...and that can be greatly affected by what you eat and drink, particularly if there's alcohol involved. I'm certainly no teatotaller, but I have seen how drinking can affect the finish on some instruments, particularly arm wear area and necks. Some people sweat out paint remover, by the looks of things. Lots of bar gigs...
This does make a pretty good argument for polyester as a finish.