Always read the instructions on the particular can of varnish or other finishing product you use on a tabletop. Similar to paint, different varnishes may require different amounts of time to set and different optimum ranges of temperatures for use. The instructions may also provide important safety guidelines and other application tips.
Varnish tends to be very smelly, and fumes may cause health problems if they are concentrated. Working in a well ventilated area helps prevent fumes from concentrating and allows them to disperse. It is also helpful to work in an area that is not used for regular living, such as a garage or dedicated work area, as the varnish smell may linger for hours or days.
Clean the work area you plan to use before applying varnish to a tabletop. If you stir up dust or other particles as you work, they may come into contact with the table and affect the table's finish. Also make sure to clear the work area of possible fire hazards such as bare wires or other sources of ignition, because varnish can be highly flammable. Similarly, ensure that the brush or other tools you use to apply varnish are clean.
Varnish is meant to be applied in several coats to get a perfectly level, translucent finish that acts as a barrier against water. Instructions may contain information on how many coats you should apply, but, as a general rule, you should apply at least two coats.