This is about as simple as a varnish recipe gets: a weak solution of a reversible resin. It should be strong enough to fill in the tiny pores and dips left by sunken-in paint, but weak enough that it does not build a real film that would stop the layer underneath from breathing and curing properly. Apply it thin, and wipe off any excess.
This recipe is worth trying if you like working the traditional way, but modern retouching varnishes (usually a weak solution of ketone resin in mineral spirits) are a solid, more convenient alternative.
Properties
Ingredients
Gum turpentine makes the varnish dry faster, but it is also a stronger solvent, so make sure the paint underneath is fully dry before applying it.
Odorless mineral spirits make it dry slower, but there is less risk of disturbing the paint film, and the varnish tends to spread a little more evenly.
How to prepare
Dissolve the resin in the solvent and mix well. Use resin in small chips or powder rather than large lumps, since it dissolves faster that way, and gentle warming of the solvent can also help. Store tightly sealed in a cool, dark place.