Other than staining my first few calls a dark brown color to make them more easily concealed in the field, I didn't apply any type of enhancing or protective finish at first. When I started making what I considered better calls, with nicer woods--calls I planned on using for many years--I wanted to apply some kind of finish to help protect against moisture and to help bring out the wood's character, but I was concerned about how a finish might affect the call's sound. As it turns out, it doesn't really affect it at all as long as it's kept off the bottom of the lid and the rails.
After seeking the advice of experienced callmakers and woodworkers, I finally settled on a formula I found in article by Garrett Hack in an older issue (February 1997, p.48-49) of Fine Woodworking magazine and involves three easy to find ingredients:
- boiled linseed oil
- varnish (I prefer satin, but any will work)
- mineral spirits
Basically, the mix involves more or less equal parts of boiled linseed oil and varnish, with a smaller amount of mineral spirits added to thin the mixture. Thinning the mix helps the wood absorb each coat more quickly and helps keep it from getting sticky or gummy. I use a small glass jar and mix just a little at a time. Since I only have to finish the top of the lid, and the outside of the sides and base, it doesn't take much, although I do use around ten coats. Using a clean rag (old t-shirts and athletic socks my wife won't let me keep), simply wipe on a layer, let it sit for a while (the longer the better, but even just a few minutes is ok, as long as the finish isn't gumming-up on the surface of the wood), then buff it out with another clean rag. Be sure to keep the finish off of the bottom of the lid and the rails!! When it's finished, let it sit in a dry, well-ventilated place for a few days if possible.
Safety Note: I follow all instructions that come with those chemicals, as they can be highly toxic and flammable. It's also important to use them in a well-ventilated area (so the fumes don't get you) and treat and dispose of used rags according to instructions, as they can spontaneously combust if left piled up. I usually rinse the rags immediately in soap and water and lay them flat outside to dry and air out.
I liked this finish immediately because it enhances the woods I use and provides some measure of protection against humidity (let's face it--box calls aren't meant for use in wet weather; if it's raining, I leave it in the vehicle or at home, anyway); besides, it's simple to mix, inexpensive, and almost impossible to screw up.
Important Note on Curing - The wood absorbs boiled linseed oil, especially when thinned as I do. While it's important to not get any finish on the sounding surfaces, in reality some finish will probably soak through the wood, particularly where the wood is thin--which, unfortunately, is the edge of the lid and the sides. A small amount of the finish that is rubbed into the top of the lid will probably soak through at the edges and appear on the bottom fo the lid (near the edges, of course); also, some finish will most likely wick up the sides and appear on the rails. This is not a problem, but it's important to inspect these areas before assembling and tuning the call, or the finish will inevitably get smeared (and, as a result, rubbed into) the call's sounding surfaces. I usually find that I have to let the finished call sit for a day or so then sand any soaked-through finish off with 220 or 320 grit paper. I also let it sit for another day or two as a bit more can still find its way through. I don't have any confidence that the finish has cured completely until it has set for at least a week with no further absorbtion.
Even if no finish appears to have soaked through, I like to give both the lid and the rails a light sanding just to be sure.
SANDPAPER NOTE: This may seem obvious, but always use a fresh, clean bit of sandpaper, especially when using it on the sounding surfaces. Used paper deposits any number of things on the wood and defeats its own purpose.