Louie Powell wrote on Oct 10
th, 2016 at 1:29pm:
Varnish - a universal finish, protects against water and handling, resists alcohol, may yellow (although may also contain UV inhibitors)
Long-oil varnish (aka Danish oil, antique oil, teak oil, etc) - combination of varnish, an oil, and a solvent. Combines features of oil and varnish
Poly (solvent) - a varnish that produces a tough, water-resistant plastic skin. Tends to yellow with age. Cleanup with a solvent (eg, mineral spirits)
Varnish ingredients typically contains boiled linseed oil (BLO), solvents, resins and/or alkyds.
A long varnish has a higher ratio of oil and works well for items that be used outdoors. It is more flexible and softer. This makes a long varnish more capable to withstand the expansion and contraction of wood movement and rain when used outside. Downside it also scratches easy which ruins the integrity of the finish so it is not good for uses such as table tops.
Spar varnish is a long oil which historically has been and still is used on boats and ships.
Originally Danish Oil (DO) consisted of equal proportions of varnish, BLO and solvent. You are in effect reducing the ratio of resins. Another thing to consider is that today commercially available DO uses soybean oil instead of BLO.
Because of all this DO does not make a good outdoor finish since it cannot handle the environment like standard long varnishes. I wouldn't call DO a long varnish.
Regular varnish and poly have less oil, dry harder, and are harder to scratch which makes them great for table tops. Both will crack and peel if the wood placed outside and is not considered a good outdoor finish.
The alkyds used in Polyurethane finishes are made from fatty acids derived from linseed oil. While some will say that Poly is a plastic finish it really isn't since alkyds are an organic compound and plastic is typically derived from non-organic petroleum.