There are many holes or lines on the over varnish surface in one of three ways:
Holes on the surface can.
Keep the overprint varnish well stirred in order to avoid the separation between solid and liquid parts.
The ink film weight is too high.
Check the can against a standard. If the shade looks stronger than standard, reduce the ink film weight by closing the fountain.
Too many laps of varnish.
Slow over varnish unit until overlap is approximately 2-3 cm.
Incompatibility between ink and over varnish.
Change ink or over varnish.
The overprint varnish is drying on the applicator roller.
Change the applicator roller.
Too much pressure on the applicator roller.
Check pressure settings.