clear, transparent
gloss, glossy
discoloured/discolored
The painting was obscured by discoloured/discolored varnish and a heavy layer of grime.

protective

coat
The table needed two coats of varnish.

apply
Apply the varnish evenly over the whole surface.

remove, scrape away, scrape off, strip, strip off

dry
protect sth
You need a good quality varnish to protect the timber.

Varnish is used with these nouns as the object:
nail, woodwork