Here is my dissertation on the different types of varnishes on the market. This will encompass most solvent based air dry varnishes.
Most varnishes on the market consist of an alkyd resin or polyurethane base, solvents, uv additives, pigments and driers. The resin base is the most important component of the varnish. It is important when selecting a varnish to read the label to determine what type of resin base is used.
Both alkyds and polyurethanes consist of an oil that is reacted with other materials to impart different properties on the resin. Oils commonly used are Soybean, Linseed, Tung, and Tall Oil Fatty Acid (comes from pine trees).
The amount of oil in the resin determines both durability and dry time:
Long Oil - greater than 55% oil - longest durability and slowest dry.
Medium Oil - 40-55% oil - not as durable as the long oils, but faster dry.
Short Oil - less than 40% oil, the least durable, fastest drying.
Polyurethanes consist of a long, medium, or short oil alkyd reacted with an isocyanate to impart better hardness of the coating.
Air dry polyurethanes are not as durable as the two component type.
You can also modify an alkyd with vinyl toluene or styrene (least durable, fastest dry). An acrylic (better durability and hardness) or an epoxy ester (fair exterior durability, better adhesion).
Spar varnishes are alkyd resin modified with a phenolic, rosin or both, and add extra oil in the formulation. These give the resin really good durability, but slow dry.
So the slower the dry, the better exterior durability.
Because all alkyds or polyurethane contain oils, they will all have a certain degree of yellowing. Those that contain tung oil, phenolic resins or rosins will be the darkest.
Water reducible versions can be made from all of the above, however they are hard to find in stores because they have limited shelf life in the can.
For more information you can go to my company's website
http://www.ranbar.com and click on the "resins" link and read the headings of the different types of resins out there.
My company supplies resins to paint and coatings manufacturers who then make paints and varnishes. We are strictly a business to business company and do not supply to the consumer. So I am not trying to sell you anything, just provide information.
If you have anymore questions please feel free to ask.