4. Nature of Various Wood Finishes Commonly Used
Linseed Oil is a natural product best described as a yellowish oil extracted from the seeds of flax grown to make linen. Linseed oil does not dry well and does little to exclude moisture from wood.
Tung Oil is also a natural product best described as a yellow or brownish oil extracted from the seeds of the tung tree. It is also called Chinawood oil. Tung oil does not dry well either and is only slightly better at moisture exclusion than linseed oil.
Boiled Linseed Oil (BLO) is a mixture of linseed oil, a carrier or solvent, and driers. It is superior to pure linseed oil in that it will more completely dry over time and is slightly more waterproof. BLO does not completely cure or dry and multiple coats dry even less well.
Boiled Tung Oil (BTO) is a mixture of tung oil, a carrier or solvent, and driers. It is superior to pure linseed oil, pure tung oil, and boiled linseed oil in that it will more likely cure or dry over time and is slightly more waterproof. The BTO finish is slightly harder and more resistant to gun oils and chemicals associated with firearms. None of these four products is really good at excluding moisture from wood.
Minwax Tung Oil Finish is an oil and varnish blend that is much more waterproof and will dry due to added driers. It can be built up into a gloss, but that is not usually done. It is best used as an in the wood finish rather than a built up on the wood finish. Minwax Tung Oil Finish resists scratches and is more resistant to gun oils and chemicals associated with firearms. The best qualities of Minwax Tung Oil Finish allow it to be wiped on and wiped off properly prepared wood leaving an in the wood finish of a look similar to BLO, but far more stable and protective. A number of coats to clean and repair scratches can be applied without building up on the surface if rubbed in carefully and then rubbed dry. Other finish products with similar sounding names may have far more varnish and behave differently in use. Some are varnishes thinned out enough with carriers and solvents so that they will penetrate into wood and not quickly build a hard film.
Birchwood-Casey Tru-Oil is a polymerized natural oil altered by chemicals and heat to produce a varnish like finish but with a few properties of oils. Tru-Oil builds a hard film on the surface of wood unless it is carefully and sparingly rubbed into the wood with none left on the surface. Tru-Oil quickly builds a gloss. Because of its hard film, Tru-Oil resists water penetration well. It also resists gun oils and chemicals associated with firearms better than any of the above products. Its chief virtues are also its chief problems. Repeated coats build up into a gloss which is often not desired.
Marine Spar Varnishes are the most waterproof and stable of the common wood finishes. They adhere well to the surface of wood and resist moisture and chemicals. As a varnish, it tends to build up a gloss and build up surface layers very quickly. Other than for a maximum effort to waterproof a stock, it has limited utility for rifle stock use.