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.

Details varnish works:
There are different types of varnish. Oil or spirit based varnish, water based varnish and acid catalyst varnish. These varnishes have different effects on the wood.
Leaves the wood as pale as possible and is very quick drying, 1-3 hours. It has a low odour, is hard wearing and easy to use, with no yellowing. Clean rollers, brushes etc. with water. Of the three is is the most environmentally safe and friendly.
ACID CATALYST VARNISHDarkens wood, is quick drying, 2-3 hours, hard wearing and has a very strong odour which can last for days. Special solvents are needed to clean brushes etc.
How to apply varnish to hardwood floors:
Floor varnish is a clear finish coat that is used to protect hardwood floors. Unlike stain, varnish does not penetrate the wood’s surface; instead it forms a seal on top of the wood. An oil-based varnish is typically used for finishing hardwood floors even though water-based varnishes are gaining in popularity because of their ability to dry faster.

The most important part of the varnish application process is preparing the floor properly. After removing all furniture from the room, the floor should be sanded down to the bare wood to remove all traces of old varnish. Any holes or cracks in the wood should be repaired with wood filler.
Always read the manufacturer’s instructions. Apply varnish with a badger hair paintbrush, china bristle paintbrush, or a sponge pad brush. A badger hair paintbrush will give the varnish the smoothest finish, while a sponge brush is easiest to use.
Varnish should be applied in a thin layer in the direction of the wood grain. Work in a small area, applying varnish to a few floorboards at a time. Varnish will be dry to the touch in about 12 hours and ready for a second coat in about 24 hours. Depending on temperature and humidity, varnish may take several weeks to completely dry. Apply a minimum of two coats. Applying a third coat is recommended but not necessary.

Oil Based Varnish:
For many amateur woodworkers or hobbyists who either cannot afford spray equipment or do not have enough space in their shop to set up a spray booth to safely spray finishes like lacquers, brushing on an oil based varnish is one of the best choices for a topcoat finish. Oil based varnish does not dry very quickly, therefore it will take longer to complete a finishing project using varnish as opposed to lacquer, water based or other faster drying finishes.