Quote:
"In a general sense, lacquer is a clear or coloured coating, that dries by solvent evaporation and often a curing process as well that produces a hard, durable finish, in any sheen level from ultra matte to high gloss and that can be further polished as required. In a narrower sense, lacquer consists of a resin dissolved in a fast-drying solvent which is a mixture of naphtha, xylene, toluene, and ketones, including acetone. The word "lacquer" comes from the lac insect (Laccifer lacca, formerly Coccus lacca), whose secretions have been historically used to make lacquer and shellac. In America today the word lacquer refers to nitrocellulose, and little else; most other coatings are known as "varnish". In the UK however, the general rule is if you spray it, it's lacquer - if you brush it, it's varnish. All factory finished furniture these days is therefore lacquer (pigmented, tinted or clear).
A polyurethane is any polymer consisting of a chain of organic units joined by urethane links. It is widely used in flexible and rigid foams, durable elastomers and high performance adhesives and sealants, fibers, seals, gaskets, condoms, carpet underlay, and hard plastic parts. Polyurethane products are often called "urethanes". They should not be confused with the specific substance urethane, also known as ethyl carbamate. Polyurethanes are not produced from ethyl carbamate, nor do they contain it.
Varnish is a transparent, hard, protective finish or film primarily used in wood finishing but also for other materials. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually glossy but may be designed to produce satin or semi-gloss sheens by the addition of "flatting" agents. Varnish has little or no color, is transparent, and has no added pigment, as opposed to paints or wood stains, which contain pigment and generally range from opaque to translucent. Varnishes are also applied over wood stains as a final step to achieve a film for gloss and protection. Some products are marketed as a combined stain and varnish.
After being applied, the film-forming substances in varnishes either harden directly, as soon as the solvent has fully evaporated, or harden after evaporation of the solvent through certain curing processes, primarily chemical reaction between oils and oxygen from the air (autoxidation) and chemical reactions between components of the varnish. Resin varnishes "dry" by evaporation of the solvent and harden almost immediately upon drying. Acrylic and waterborne varnishes "dry" upon evaporation of the water but experience an extended curing period. Oil, polyurethane, and epoxy varnishes remain liquid even after evaporation of the solvent but quickly begin to cure, undergoing successive stages from liquid or syrupy, to gummy, to tacky, to "dry to the touch", to hard. Environmental factors such as heat and humidity play a very large role in the drying and curing times of varnishes. In classic varnish the cure rate depends on the type of oil used and, to some extent, on the ratio of oil to resin. The drying and curing time of all varnishes may be speeded up by exposure to an energy source such as sunlight or heat. Other than acrylic and waterborne types, all varnishes are highly flammable in their liquid state due to the presence of flammable solvents and oils."
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On the mention of blooming; that is actually quite apt as there is a phenomena whereby certain fast curing finishes including nitro, solvent acrylic and 2pack when applied at below required temperatures "bloom" - you get patchy bits that are matt finish, the curing procedure, adhesive properties and durability as far as I've encountered isn't affected to any great degree.
Spar or Yacht varnish is primarily designed for outdoor use with a higher content of oil giving a more flexible finish to allow the wood to expand and contract with outside conditions.
Nitro cellulose (apart from requiring a hazard mask to spray) is harder wearing per application that many other finishes with higher abrasion and much better water resistance; is fast drying initially and can be recoated in that time, but takes time to fully cure full strength. Known mainly for being used on instruments, but is becoming more common in other areas where it's properties are an advantage to manufacturing and use.
Acrylic is usually water based, but some rattlecan acrylics (like car varnish) are solvent based and still have a fast initial curing period. The water based finish doesn't usually penetrate as well as oil based, and can increase the "nibbing" effect of some woods, so a pretreatment of water and denib first is recommended. They also don't apply or self level as well as oils, partly because of the reduced drying time compared to oils, and partly because of the fact is it water based.
2part or 2pack finishes are essentially thinned epoxy resins tinted (or not) for colour - spraying is the only real method to apply these and again a hazard mask (full air fed) is an absolute must if you do it regularly.
Unlikely you'll get to use UV catalysed finishes so I'll skip that.
most of that was my own experiences but for completeness here's a wiki:
https://en.wikipedia.org/wiki/Varnish#Acrylic
