Broken Ruger's advice to buy that book is spot on. I got about the same kind of knowledge from purchasing five different issues or special editions of Fine Woodworking Magazine from Amazon. Cross referencing the information and definitions with my library of custom stockmaking, custom gunsmithing books, etc will lead you to the basic ideas.
My interests are in fine English or Scottish doubles. One of my second or third quality hammer guns is a T. Wild 3" , 30" waterfowl double. Using it late season in the snow and sheet caused the wood to discolor alarmingly, but not under the lace on pad where it was dry. It reverted back to normal after several months inside. The wood finish I suspect was simple boiled linseed oil diluted 50/50 with mineral spirits. It is definitely not waterproof. Tung oil finish is not pure tung oil which is a bit more water resistant but not much.
Most high end techniques will start with several treatments of sealer which WILL waterproof the wood. My only experience is with three x of Permayln Sealer followed by applying Permalyn Finish worked in with progressively finer wet and dry sandpaper from 200 to 320 to 400 to 600 to fill the pores if that is your goal. Then, just a drop of Permalyn here and there rubbed in with the base off your thumb/palm another ten times. These processes can take twenty to thirty hours on a long gun.
Revolver grips less of course. This finish is not hard, plastic looking,or scratch proof but scratches are easily repairable back to original. There is not much wrong with Tru Oil or other products either....just follow the directions and let each treatment dry very completely or you can ruin your project early on.
Those of us who enjoy handling our firearms.....most of us.. would enjoy the learning and the labor.