Fast Tip: Shaken not Stirred Varnishes
Much like a classic Martini, there's a misconception that varnishes should not be shaken or stirred prior to painting. Shaking a varnish can cause air bubbles in the can which will take time to dissipate. Although there's little in gloss varnishes to settle, flat, satin and semi-gloss varnishes contain a pigment that may settle out.
Avoid varnishes in your low-gloss varnish (as you get lower in the tin) by stirring well before using.