The two most common are excessive moisture in the area, which applies with both spray and brush on varnish. This could be due to high humidity, the previous layer of paint not being fully dry or in the case of brush on varnish, the brush itself not being dry.
Milky or Cloudy Varnish: If your varnish has not dried totally clear - It's cloudy or milky; then the simple answer is that it is not dry underneath. Water based varnishes dry incredibly quickly. If you put them on too thickly the top layers of paint will be dry to the touch.
Varnish goes cloudy if too much or wrong temperature. Try removing varnish with Acetone (careful flamable) and then rubbing in Dorlands cold wax medium or spray lightly with Montana 94 acrylic topcoat.
What this whiteness is, is actually microscopic air bubbles.
Applying too much varnish in one layer or letting it pool up leads to cloudy, discolored spots such as in the image below. This phenomenon is more noticeable with matte varnishes than it is with gloss varnishes but should be avoided regardless of the varnish you are using.
Incorrect clear coat application technique
Applying the first clear coat layer with excessive pressure or at too short a distance from the part can cause the aluminium particles to be removed from the surface, which can lead to the appearance of clouding or cloudiness.
To fix this, the surface needs to be scuffed and sanded first. Second, we recommend applying a sanding sealer to protect the top. Sanding sealer has a larger resin particle, so it will provide a better build for the next sanding step. Then finish with your topcoat.
“All that white mark is,” said Gat Caperton, “is trapped moisture. The finish on your furniture is there to prevent the exchange of moisture from the air to the wood. That hot item softened the finish enough for it to separate from the wood.
Sand the surface smooth and flat before applying the final coat. Do your best to eliminate all the dust you can for this last coat. If you have done everything else well, your finish will look great even with a little dust. If the remaining dust bothers you, buff it out later.
If you are left with a sticky layer of wood varnish that won't dry, there is a simple fix to put it right. All you need is some white spirit and a dry cloth or an old t-shirt rag. Start the process by wetting the rag with a generous amount of white spirit.
Sanding Tips (cont'd.): If you've applied 2-3 coats in one day, allow 48 hours drying time, then sand lightly, with the grain, using 320-grit sandpaper, before overcoating or applying the final coat. If 1 coat has been applied, the surface can be sanded within 24 hours.
Varnish finish
Some varnish finishes can be reconditioned with a mixture of 3 parts boiled linseed oil and 1 part turpentine. Mix this solution well and apply it sparingly to the varnish with a cloth or grade 000 steel wool. This may work - it's worth a try.
Cloudiness or White Spots
To fix this: Allow the finish to dry completely. Lightly sand the cloudy areas with fine-grit sandpaper. Reapply a thin coat of polyurethane, ensuring proper drying conditions.
Often, the beginning stages of clear coat failure appear as subtly dull or white patches on the horizontal surfaces of a vehicle. As the oxidation process continues, paint eventually starts to crack and flake (see the picture above.) Sometimes paint failure is so severe it looks like skin shed by a snake.
If the Top Coat becomes cloudy, it may have been contaminated with a white pigment or with water.
The more exposure clear coats get to weather, the faster they wear out. If you're living in a frigid environment, clear coats can fade or appear chalky. If you're living in a hot environment, clear coats can fade or appear to be dull and scratched up with time.
Ensure that the surface is clean, smooth, and free from dust. Use a high-quality brush to apply the varnish in thin, even coats, and always follow the manufacturer's instructions for drying times between coats. Sand lightly between coats to ensure a smooth finish.
If you do decide to go ahead and apply new varnish over the top of old varnish, make sure that the existing varnish is scratch-free and do give the old varnish a light sanding using 120-grit sandpaper and wiping away any dust. This will ensure that the new varnish has something to bond to.
The number of coats you apply depends on both the type of varnish you're working with and the finish you want to achieve. Applying more than two coats of matt or satin varnish could mean making your painting look cloudy, but with gloss varnish you can apply up to three coats.