How to waterproof wood?
The 3 best waterproof finishes are varnish, polyurethane, and lacquer. Oil-based varnish is composed of resin, which cures to a film finish that seals the wood and protects it from water. While polyurethane acts the same way, it is made of polymers, which are similar to plastic. Lastly, there is a lacquer, which when made synthetically contains polymers, like polyurethane.
All three of the above finishes are synthetic in nature, in the case of varnish, it is usually mixed with naturally processed oils that are absorbed by the wood. All of these finishes dry to a clear film finish, there might be some hue due to the chemical processing. They are glossy and transparent which does not block the view of the beautiful wood grain.
The way oil-based varnish works is pretty much like any other synthetic finish. The oil in the varnish is absorbed by the pores in the wood and the resin penetrates only on the surface. Once the finish is cured, the resin is hardened sufficiently and creates a clear coat that is durable. While varnish dries fast, it takes a similar time as other synthetic finishes to cure.
Spar or marine varnish seals wood like no other finish, which is why wooden boats from head to toe are coated with it and left to cure for weeks. After which it is protected from prolonged exposure to water for a year or two before you need to apply another coat.
The metal drier in varnish can be toxic so it is not a food-safe finish which in turn means it cannot be used in the kitchen. Though the metal driers usually evaporate with the fumes when the varnish is curing it is still wise to avoid it altogether in case some traces are left on the coat.
These days most people prefer water-based polyurethanes as they dry really fast. But as the wood absorbs water, the grain raises a bit which needs to be knocked down before applying another layer. Oil-based polyurethanes on the other hand are better at layering but take a lot of time to dry and cure, which might be a deterring factor.
Also, the quality of the polyurethane mostly depends on the kind of polymer resins present in it. Since polymers have different grades, some polyurethane finishes look better compared to others. Cheaper ones have a more plastic feel and look to them. While the more expensive brands will give you a better look and feel depending on the grade of the polymer present in it.
Polyurethane is the most used finish these days because of its quick drying times when it contains water solvents. It is less flexible than varnish, so it is great for sealing static objects like flooring, etc that are static.
Lacquers consist of synthetic polymers like acrylic resin and lacquer thinners, which are solvents. Once it is applied to any wooden surface it hardens very fast. Though lacquer consists of VOCs and should only be applied in an open space and with a respirator on. If you have any pre-existing breathing conditions, it is advised to not apply lacquer at all.
Lacquer not only dries fast, but it also cures faster than any other finish. It hardly takes 7-10 days for lacquer to cure to its full hardness. This means that all the chemicals that are in the mix, solvents, will have evaporated by then and only a layer of synthetic polymer remains to coat the surface of the wood.
If you are spraying on lacquer, you will need to apply at least 3-5 coats for a durable finish. Once it is fully cured, it will seal the wood completely and make it waterproof. The only issue with lacquer is that it is more brittle than the other two finishes mentioned above. So it cannot take a lot of abuse and is not flexible so should not be applied to any projects that might require flexing.