That is a very interesting question Uncle Alva. The answer is: it depends.
But, first of all, linseed oil and turpentine is a poor gun finish. It will take months to dry, and when it does, you will have a finish that is not particularly weatherproof, and will require regular maintenance. At a minimum, you will need to add a metal drier so that it dries in a reasonable amount of time. The traditional drier is lead carbonate, which is cooked into the linseed oil / turpentine mix. This is boiled linseed oil, or BLO. Instead of lead you can also use Japan drier, which is cobalt and manganese. Japan drier will give you a similar, but inferior product.
You can get a much better finish by adding a resin to the linseed oil. The simplest is pine resin, which makes what we call spar varnish or Danish oil. Other recipes have additional resins, such as mastic, amber or gum benzoin.
So, back to your question. Briefly, you can start with raw linseed oil and refine it yourself. Or you can start with a refined oil specifically intended for making varnish. This is a good source for refined linseed oil
http://www.woodfinishingenterprises.com/coating.html You can get raw linseed oil at the health food store.
Other grades of linseed oil will also work. Same with the turpentine. You can use hardware store turpentine, or a good turpentine like Grumbacher’s. As with other things, the better the components, the easier they are to work with.
There are a lot of threads on this site that discuss making linseed oil finishes. Use the search function and you’ll find them. This is also a very good place to start:
http://www.muzzleblasts.com/archives/vol5no2/articles/mbo52-1.shtml
