Bought a painters knife the other day to try for applying finish (it's shaped like a pie server with a flat tip). It works great. Very thin so can turn on edge to apply finish right along threads to get a nice straight line in the finish. Also, for long butt wraps, inlays etc... you can use the upper or wider portion of the blade to apply finish evenly over a large area. I believe I bought a #4 at Hobby Lobby or Michael's for $4.95. Works great.

Sounds like you bought a pallette knife. These are flexible tools used for mixing artist paint. They look like a little cement trowel, the kind with an offset handle. They come in both metal and plastic with a very wide variety of shapes and strengths. They are great for mixing epoxy. You will find them by the artist brushes in any craft or art supply store.