Varnish is a transparent, hard, protective film or finish used on the surface of wood. It is traditionally a combination of drying oil, resin, and a solvent. Varnishes are usually glossy, but can be modified with flattening agents to produce flat or semi-gloss sheen.
Varnish is a vast encompassing category that includes: shellac, polyurethane, and lacquers. But for this pages sake we will be focusing on conversion varnish and spar varnish.
Conversion varnish, or sometimes known as catalyzed varnish, is a two part based finish. The first part is resin based and the second is an acid catalyst you add just before spraying. Always follow the manufactures directions for mixing the two parts.
This type of finish must be sprayed on, and dries very fast after being applied. It produces a beautiful and durable finish that can stand up to heavy abuse.
Conversion varnish is for professional use only and you will not see it in your local paint store or home hardware store. It takes a specialized paint both to spray since you need warm temperatures and a proper air movement system. This varnish will produce formaldehyde when it is curing. Also it takes a special hand to apply, because too thick of a coat can cause it to wrinkle and crack.
Spar varnish was originally developed for coating the spars on sail ships which suffered a hard life. They were flexed by strong winds, hit with bad weather and sea water, and also suffered from UV degradation. So it was important for this varnish to flexible and elastic. Nowadays spar varnish refers to any outdoor varnish and the principles of flexible and elastic have long been forgotten.
The modern day spar varnish protects the wood surface from weather and sunlight. As well offering protection from water and alcohol.
To apply spar varnish you follow the same principles as most finishes. Make sure the wood is free from oils, grease, dust, and debris. Use a pure bristle or polyester brush, flat pad or spray. Apply in full even coats with minimal brushing to achieve a glass like finish. It dries to touch in 4 to 6 hours and wait at least 8 to 16 hours to recoat.