If you have applied wood varnish to a piece of furniture and now the surface is cloudy, there are several possible reasons for this.
Reason 1: Wet Surface
If you apply a coat of varnish before the stain is totally dry, it may become cloudy because it has sealed moisture under it. Always wait at least 24 hours between staining and then varnishing a piece of furniture.
Reason 2: Old Varnish
If you use varnish that has been sitting very long or wasn't sealed tightly after a previous use, it may be too thick. When applied, it may not dry completely and may instead achieve a cloudy, sticky appearance. Always seal your containers tightly after use. If varnish has been on the shelf more than a couple of years, you may want to discard it.
Reason 3: Second Coat
If you apply the second coat of varnish over the first coat too soon, you may get clouding. This is caused from trapped moisture. If you leave it to dry, it may eventually clear.
Reason 4: Humidity
If you are using oil-based varnish and the humidity is high, you may get clouding. Again, this is caused by moisture. Moisture is the enemy of freshly applied varnish.