MIMF

Hi,

What is a primer and why is it necessary to apply before lacquer? This is to finish a classical guitar.

Thanks

No.

Unless you need a filler coat, i.e. not a primer but something to fill the open pored grain in certain woods like mahogany, walnut, East Indian Rosewood and many others.

Primer/sealers reduce the number of top coats that need to be applied to obtain a smooth finish. You can "self seal", which is to say spray additional coats of top coat until it fills the pores of the wood.
Some guitar builders use epoxy resins to fill the pores of the wood and top coat over that.

A primer is a base coat usually employed to improve adhesion between the surface and the top coat. A sealer is similar, but it's purpose is to seal the surface. A pore fill is a little different, it's purpose is to seal the pores in wood that has pores. Primers are not generally used on wood, while pore fill and a seal coat are common under lacquer. Classical guitars traditionally have a French polish finish, which is a hand applied spirit varnish.