A number of years ago I began to finish the inside back and sides of most of my acoustic instruments. I use natural shellac for the interior of my instruments. I find that the finished interiors make the instruments much more dimensionally stable during temperature and humidity changes. It also makes them look rather fetching. At one point I thought about sizing the inside with thinned hot hide glue in the manner of some violin makers. I decided against this practice for myself as I feel it might create clamping problems for any future repairs as the slighest bit of moisture would grab the clamp face and create unnecessary fuss and drama for the repair person. Shellac has worked great for me for years now and I love that it is non-toxic! I got you covered even on the inside!