Excerpted from my book. If you quote this, please give proper attribution.
The Physiology of a Good Finish
There are three components of any good finish: base coat (s); build coat (s); and top coat (s).
* The Base Coat – The Most Important
(Shellac, sanding sealer, friction finish, CA, etc.)
First, before reaching for any finishing product, it is critical to have a well-prepared surface. The best finish in the world cannot overcome a poorly prepared blank! Sanding, grain filling, and defect filling are of paramount importance prior to beginning the finish process.
The first (or only, in some cases) coat on a new wood surface should seal and even out the porosity and the density. On soft woods and end grain, multiple coats are desired in order to provide an even base for the build coat (s). Any unevenness needs to be addressed with additional base coats. The build coats depend on it!
* The Build Coats
(CA, CA/BLO, lacquer, polyurethane, etc.)
In general terms, the build coat is where we introduce moisture resistance, durability and depth of the finish. The base coat may provide some of these, but its primary purpose is to prepare the wood for the build coats.
Build coats should be gloss only. Anything else reduces clarity of the structure, grain and tone of the wood. In almost all products, the glossy version is the hardest. That's critical in build coats. Do not confuse this glossiness of the build coats with the final appearance, which is controlled by the top coat.
The number of build coats is dependent on two factors – the desired depth of finish and the softness of the wood. Softer wood requires more coats to provide overall moisture-resistance and durability. Durability of finish increases geometrically with the number of coats applied. Depth of finish is easy—more coats equal more depth!
* The Top Coat – The Ooohhh Factor
(Paste wax, Renaissance Wax, buffing compounds, TSW, etc.)
Many of us rush to this step as we are looking for the gratification of bringing to life that which we originally envisioned. Cutting short the two prior steps will insure that this step does not produce the original desire.
The top coat is almost all about esthetics. Matte, satin, semi-gloss and gloss sheens are imparted in this step. Surface preparation comes to the front again. The surface must be "roughed up" a bit in order to provide proper adhesion to the build coat (s). After light sanding, the surface must be cleaned thoroughly. Careful removal of dust and contaminants will save lots of time and aggravation. Pressurized air is one of the best means of cleaning the surface. If you're like me and don't have an air compressor, use the dust blower cans that are intended for cleaning out computers and electronics.
Since most of our finishes require buffing or polishing, it is critical that the prior layers of finish are completely dry and, for the most part, cured. This is the hard part since most lacquers take 2-3 weeks to fully cure and shellac (and friction polishes) can take 30 days or longer. In any case, one of the worst things you can do is rip the just-finished pen off the mandrel and begin assembly. Waiting even a few days will make a remarkable difference in how the end result looks and wears.
How CA Works
CA or cyanoacrylate is acrylic resin that cures (forms its strongest bond) almost instantly. The only trigger it requires is the hydroxyl ions in water, which is convenient since virtually any object you might wish to glue will have at least trace amounts of water on its surface. [NOTE: This convenience can become a disadvantage if you are using CA in a highly humid environment.] Cyanoacrylate molecules link up when they come into contact with water, and they whip around in chains to form a durable plastic mesh. The glue thickens and hardens until the thrashing molecular strands can no longer move. Cyanoacrylate glues are moderately toxic by skin or eye contact. They can glue the skin together or glue the skin and other materials together, sometimes requiring surgical separation. Acetone can be used to break these bonds. Eye contact can cause severe eye irritation. Their long term hazards are not well studied, especially with respect to inhalation.
Boiled linseed oil (BLO) is sometimes used in conjunction with CA in finishing. The primary purpose of the BLO is as a lubricant which allows you to level the surface. The accelerant action is really secondary. The BLO does not "mix" with the CA and does not form another layer or strata in your finish. In fact, excess BLO should be wiped off before each re-application of CA.