Dear Ira,
Marine spar varnish and Spar varnish are the same thing. They are varnishes specifically formulated with phenolic resins and oils to protect in exterior uses, especially on surfaces which must have flexibility, ie. boats and ships., and must withstand strong wind forces.
Here is a quote from the article below:
Phenolic resins combined with tung oil produce a varnish with superior water resistance, good hardness, exceptional flexibility and good alkali, grease and UV resistance. It's called spar varnish. Unfortunately, the drying times are excruciatingly long, and spar varnish has a deep yellow color that only gets worse as it ages. Still, spar varnish is the best choice for projects, such as outdoor furniture, that will be exposed to the elements.
Here is an article from Fine Woodworking magazine on the subject.
Varnish: An Almost Ideal Finish
Understanding the chemistry behind many common oil varnishes helps you choose the right product
by Chris Minick
My ideal furniture finish would penetrate deep into the wood, dry quickly, provide good abrasion and stain resistance, rub out easily and look great. Also, I want the option to apply this finish with a rag or a brush or a spray gun. Unfortunately, no finish has all of these properties, but oil-based varnish comes pretty close.
Walk into any well-stocked hardware store, and you'll find a bewildering array of cans of varnish. Alkyd, polyurethane, spar, wiping, bar-top, floor, fast-drying, interior and exterior varnishes are the more common types available. With all of these choices, selecting one that's right for your project can be frustrating. Knowing a little about varnish chemistry may help you decide.
They're more alike than different
All varnishes have one thing in common: The backbone of the finish molecule is composed of vegetable oil. Varnishes are made by chemically combining a modifying resin with a vegetable oil to produce a finish molecule that is liquid when applied to the wood surface but solid after it cures to an impervious film in a short period of time. Linseed oil and soybean oil are the most common vegetable oils used in the manufacture of furniture-grade varnish resins. Tung oil is also used, but because it is relatively expensive, you find it only in wipe-on varnish mixes and traditional exterior varnishes. The type of oil used in a varnish resin has less effect on the finish properties than does the amount of oil used.
The long and short of it -- The ratio of oil to modifying resin--known as oil length in the industry vernacular -- determines the flexibility of the dried film, curing or drying time and application method. Varnish resins containing 75% or more oil are called very long-oil varnishes, and they're typically used for wipe-on finishes, the so-called Danish oil finishes. Very long-oil varnishes dry slowly, have great wood penetration and are extremely flexible when cured; but the dried finish film is extremely soft, has poor abrasion resistance and damages easily. These varnishes perform adequately, provided that no detectable finish film is left on the surface of the wood.
Long-oil varnishes are 60% to 75% oil and are primarily used in the manufacture of oil-based paint but recently have been introduced to the wood-finishing arena as fast-dry wipe-on finishes. Long-oil varnishes do dry faster than traditional Danish oil finishes. However, they share many of the same problems that plague their very long-oil brethren.
Medium-oil varnishes contain 45% to 60% oil and form the basis of all brush-on varnishes used in wood finishing. They have reasonably short drying times, good abrasion and stain resistance, penetrate the wood to accentuate its beauty and form a hard but flexible protective surface film. Best of all, medium-oil varnishes can be applied by the three most commonly used methods: wiping, spraying or brushing.
Short-oil varnishes are less than 45% oil and typically require heat to cure, so they are not used for finishing wood. Short-oil varnish resins are used to make the paint for refrigerators, stoves and metal office furniture.
Alkyd varnish.
Once the mainstay of the furniture-finishing trades, this class of varnish is increasingly hard to find.
Polyurethane varnish. Polyurethane dries fairly quickly and exhibits good moisture resistance, but it does not hold up well to the degradation brought on by ultraviolet light.
Fast-dry varnish.
The vinyl toluene and naphtha listed on the label identify this as a fast-drying finish.
CAS No.
Ingredient
64742-89-8
V.M. & P. Naphtha
Unknown
Vinyl Toluene-
Oil Polymer
64742-88-7
Mineral Spirits
1330-20-7
Xylene
100-41-4
Ethylbenzene
Spar varnish.
Tung-oil-based phenolic resins are found in most brands of spar varnish, and they are the ingredients that help this product stand up to the elements.
Ingredient
Tung Oil Phenolic
Resin
Alkyd Resin
Stoddard Solvent
Mineral Spirits
Dipentene
Modifying resins vary in strength -- The modifying resin used in a varnish will determine how well the dried finish film holds up when it is exposed to moisture, ultraviolet (UV) light and general wear and tear. Alkyd varnishes, a term coined in the 1930s to describe an important class of polyesters, traditionally use phthalic anhydride as the modifying resin. Typical alkyd varnishes have good flexibility, very good abrasion resistance, great adhesion, moderate moisture resistance and take a relatively long time to dry and cure (8 to 10 hours to dry to the touch and 16 to 20 hours to re-coat). Alkyd varnishes targeted for furniture making are difficult to find these days, but they remain a mainstay of hardwood floor finishing.
Replacing some or all of the phthalic anhydride with toluene diisocyanate yields the familiar polyurethane varnish, which is also called uralkyd by finish chemists. This modification to make polyurethane decreases the drying and curing times to more tolerable levels and increases the moisture resistance of the finish film, but it sacrifices the UV resistance in the process.
Fast-dry varnish, also called VT varnish, uses styrene or vinyl toluene as the modifying resin to produce a product with remarkably fast drying times. Properly formulated VT varnishes will dry and cure almost as quickly as nitrocellulose lacquer--dry to the touch in 30 minutes and cure to re-coat in less than two hours. However, fast-dry varnishes have slightly less protective properties than standard alkyd varnishes. Still, they are suitable for most furniture applications. But I would not recommend them for heavily used kitchen or dining-room tabletops.
Phenolic resins combined with tung oil produce a varnish with superior water resistance, good hardness, exceptional flexibility and good alkali, grease and UV resistance. It's called spar varnish. Unfortunately, the drying times are excruciatingly long, and spar varnish has a deep yellow color that only gets worse as it ages. Still, spar varnish is the best choice for projects, such as outdoor furniture, that will be exposed to the elements.
Most people choose to brush it on
You can apply varnish with a brush, a rag or a spray gun. Wiping it on is definitely the easiest method, and spraying is the fastest. But brushing is the time-honored technique for applying varnish.
Your success with a brushed-on varnish will depend on practice and attention to detail. I've found that most varnishes are too thick to use right out of the can, so I thin them to about the consistency of whole milk. Prior to dipping the brush into the varnish, you should prewet the bristles with the same solvent used to thin the finish. Prewetting conditions the bristles and prevents the buildup of dried finish at the base of the brush, making it easier to clean later. Shake out the excess thinner, then fill the brush by dipping it into the thinned varnish by no more than half the bristle length. Capillary action will automatically fill the brush reservoir with the proper amount of finish. Tap the bristles on the inside of the can to remove the excess varnish, and always finish the unseen areas of the project first, such as the inside of cases or the underside of tables. You'll be able to judge the flow and leveling properties of the varnish before tackling the show side of the piece. If the viscosity doesn't seem right, add varnish or solvent.
China bristle brush
You don't want a big brush for varnish. A 2-in. or 2-1/2-in. china bristle brush works best for laying a varnish onto flat surfaces. Photo: Michael Pekovich.
Most finishing projects require more than one brush. I use a 1-in. ox-bristle sash brush for coating small or intricate areas such as moldings and spindles. A good-quality 2-in. china bristle brush is ideal for large, flat areas. Larger brushes are inappropriate for furniture finishing because they're just too hard to control.
Avoid the choppy, back-and-forth stroke used to apply house paint --i t will result in an uneven surface and lots of bubbles. Instead, slowly pull the brush across the panel in one continuous motion until the brush reservoir is empty. Hold the brush at about a 45° angle when it first contacts the surface and gradually increase the angle to almost 90° by the end of the stroke. As this bristle angle increases, more varnish is released from the reservoir and flows to the wood surface. I usually get a stroke length of about 18 in. or 20 in. with a fully loaded 2-in. brush.
After the entire surface has been coated, "tip off" the varnish by lightly dragging the bristle tips through the wet finish. Tipping off with an unloaded brush levels out the uneven areas in the wet varnish film and removes unwanted bubbles at the same time. You don't need to sand between coats of varnish unless you have to remove some defect. Simply sanding to increase adhesion is not necessary unless the dried varnish coat is older than six months.
Avoid the pitfalls of using varnish -- Extended drying time under certain conditions is the biggest problem I've experienced with oil-based varnishes. High humidity drastically prolongs the drying time of most varnishes. Anybody that has varnished during the dog days of summer knows just what I'm talking about. I avoid varnishing on those dripping-wet days, if possible; if not, I let a dehumidifier run in my shop for a few days beforehand.
Also, old varnish dries more slowly than fresh varnish. This too has to do with the metallic driers that gradually lose their catalytic powers as the varnish ages. The best way to avoid the problem is to make sure you always use only fresh varnish. Typically, I don't use varnish that is more than one year old. I may waste a few dollars, but I save a mint in frustration.
Chris Minick is a contributing editor to Fine Woodworking magazine.
Photos except where noted: William Duckworth
Not hijacking just borrowing 
