When heating the Iron Nitrate, how hot does the wood's surface need to be?
Use a heat gun on medium setting. The wood's surface goes from a greenish gray to a warm orange color. You will see it change. Make sure there is no greenish gray left and that the whole surface is blushed. Be careful with thin parts of the rifle because they heat up faster than the rest and can scorch. Scorching is not the end of the world. If it happens, sand it back a little and reapply iron nitrate. Keep the heat gun moving in small circles to help avoid scorching. This process of going from gray brown to warm orange brown is called blushing.