The usual finishes that are sprayed are lacquer, shellac and water-based finish. These finishes dry fast, sometimes too fast in warm temperatures to successfully brush onto large surfaces. Spraying overcomes this problem.
Oil-based varnish, including polyurethane varnish and oil paint, can also be sprayed, of course, because any thin liquid can be sprayed. But you need to be aware of a significant difference. Varnish dries much slower than the other finishes, and unlike lacquer, shellac and water-based finish, each coat should be allowed to fully dry before the next coat is sprayed.
You may typically spray lacquer, for example, spray several coats at a time without allowing drying in between. The drying will be a little slower because there is more solvent in the thicker application that has to evaporate for the finish to harden. But the time difference won’t be great, and there won’t be any problems unless you build the finish so rapidly that it sags or puddles.
Varnish and polyurethane varnish (and oil paint) are different. For varnish to cure, oxygen has to work its way into the finish to cause it to crosslink. If you spray several coats of varnish one after another, the top surface will dry well before the varnish at the bottom, and this will significantly slow the penetration of the oxygen. The result will be a finish that feels dry at the surface but is still so soft that you can indent it with your fingernail, and it will remain like this for many days or weeks.
You can always allow way more time for the finish to cure all the way through. You can also speed up the curing by raising the heat in the room or applying heat with a heat lamp.
But this could cause the finish at the surface to wrinkle as the finish at the bottom dries and shrinks.
So it’s best to allow each sprayed coat of varnish or oil paint to fully dry (enough so sanding produces dust) before applying the next coat.