A well-applied brushed finish is arguably better than a badly-applied sprayed one. Unless you use cans, spraying a guitar always requires a lot of preparation, a suitable room and expensive equipment. You should really seriously ask yourself if a brushed finish wouldn't also meet your own personal demands, especially because they require far less in the way of preparation and are also much cheaper. For only a small part of the money you would otherwise have to spend on spraying equipment you can already buy the best brushes available on the market. I consider brushed-on varnish to be a very good alternative to spraying lacquer.
Varnish is difficult to spray and is therefore mostly applied by brush. It is far more durable than a pure oil finish. Bar-room tables, for instance, are varnish-finished.
Varnish is basically oil that has been cooked with natural resins such as rosin, amber or copal. Depending on the oil-to-resin ratio the results will be harder or softer. This is how, for example, violin varnish is produced. Nowadays polyurethane is commonly used in place of natural resins. Varnish also cures coat by coat and does not bond with coats applied underneath. Since it takes a long time to cure, an absolutely dust-free environment is essential. There are special brushes with bristles forming a tapered, chisel-edged end which are particularly suited for applying varnish.
Use turpentine for thinning natural resin varnishes. Mineral spirits, which are made from petroleum, are often not even suitable for cleaning brushes.
All coats of varnish, apart from the first, should be applied v-e-e-ry slowly and should rather flow off the brush than actually be brushed on. If you are interested in a demonstration of how to correctly apply varnish, I can recommend the video Hand-Applied Finishes: Applying Top Coats by Jeff Jewitt and also the book Hand-Applied Finishes by the same author.
By thinning varnish with mineral spirits or turpentine you get what is called "wiping varnish", which is easy to apply with a cloth.
Excerpt from my book Building Electric Guitars