If you are planning to use polyurethane sealants to put a glossy sheen on your wood project you may be having trouble deciding which route to take with regard to what kind of covering to choose. If it is a question of Polyurethane Sealants versus natural resin varnish then here is a guide that might help.
Varnish is made by a process of mixing drying oils (such as linseed or walnut), solvent and natural resins to produce the shiny and glossy effects that it can offer to your wood project. Varnish usually has a high ratio of oil to resin. The drying process of varnish is caused mainly by the chemical reaction of the drying oils mixing with oxygen, in other words, the evaporation of the solvent within the varnish. There is also the water based versions, which offer similar finishes, but the lack of gloss in the final finish will not be as high as with oils.
Polyurethanes are a mix of oils synthetic resins. They offer abrasion-resistant and durable coatings. The drying process is similar, in that it is still a reaction between the solvent and oxygen. If you compare them to simple oil or shellac varnishes, polyurethane varnishes do form a harder, tougher and more waterproof cover.
Polyurethane sealants are highly durable for hardwood flooring surfaces. However, of you use it on such a floor and you coat it too thick it may react badly if subjected to high heat. The tendency to break down will increase with long periods of exposure to sunlight, also, if you apply it to softer woods, such as pine. However, very slow drying time allows for particles of dust to settle on the finished surface and these spores stick into the partly set coating, leaving a slight texture in what should be a clear sheen.
Varnish offers high durability to any surface. The application of varnish gives a highly effective gloss finish. Again, it has slow drying time and each coat that you apply is very distinctive from the previous coat applied. When the first coat dries it will leave a mark, not a visible mark so to speak, but a sort of template. When you apply the next coat on the top it will leave a slightly different template. Because of this layer effect it makes sanding difficult, as it will expose any blemishes to top surface.
So, the upside of using varnish is that you get durability and hard wearing. Also, high water resistance and high gloss finishes. The downside of using varnish is that it has a very slow drying time. The layer effect of each coat can effectively detract from the grain of the wood, leaving its own template marks. Most varnishes will ‘yellow’ after time, causing a slight aging appearance to the wood.
Polyurethane sealant also has a very slow drying time, but there is no layering effect as with varnish but similar disadvantages do apply. Dust will still form and leave a gritty surface and can leave blemishes in the first coat, which will translate through to the very top coat if not sanded out correctly.
Effectively, in comparing polyurethane sealants with natural resin varnishes, it really comes down to a matter of choosing the right sealant or varnish for the right project. Sometimes natural oils are better and other times polyurethane sealant will be more appropriate. Make sure you always choose the right coating for the right finish.