Varnishing

Varnish protects the surface of a painting from dust and other airborne pollutants. When paint dries, small pores open up in the surface to let the solvent escape; these pores make the surface vulnerable, and that is why a coat of varnish is required after the painting has dried completely.

From left: Spray varnish, liquid varnish and brush.

Note: I will always take paintings back to varnish them free of charge.

Acrylics 

Acrylic paint is water-based, and dries quickly. This means that even an acrylic knife-painting, with its thick paint, can be varnished within a week or two of completion. I don't usually let acrylic paintings out of my possession without applying varnish.

Oil Paintings

Varnish cannot be applied to an oil painting if the paint has not dried thoroughly. Oils dry extremely slowly. Even if the surface is dry to the touch, the paint below it may still be drying out. The general advice is to varnish after 6 to 12 months, depending on the thickness of the paint. As I tend to apply thick wodges of neat paint to the canvas with a knife, it is fairly safe to say that the paint will take a very long time to dry properly, and so I would not recommend varnishing before a year has passed. A painting can, however, be framed and displayed in this time.

All varnishes for oils can be removed at a later date.

Advice on Varnishing Impasto Paintings

There are two approaches to applying varnish: you can either spray it on, or you can paint it on, with a brush. The difficulty with impasto paintings (ones with an uneven surface, which includes my knife paintings) is getting the varnish into every nook and cranny.

Whatever you do, always use varnish formulated for artwork; household varnish, or yacht varnish, is not suitable! Gloss and matt varnish is available in both spray and liquid form.

Spray Varnish

On the face of it, this seems like the easiest option, and I suppose that it is. But you have less control over the application, and if there are places where the paint overhangs itself, a spray may not provide adequate coverage.

Traditional Varnish, applied with a Brush 

The brush should be a largish flat hog brush; you can buy special ones for varnishing.

The standard advice is to make sure the painting is completely dry (we already did that, I think), and then to clean it with water and cotton wool. (The cotton wool gets caught on the pointy bits of paint.) After you've removed all of the cotton fibres, leave the painting to dry, leaning face-in against a wall, for several hours.

Lay the painting flat and apply the first coat of varnish thinly, in parallel strokes, always working in the same direction. Leave the painting flat for at least 10 minutes before you prop it up, face-in, against the wall again to dry. It will take one or two days to dry, then you can apply the second coat at right angles to the first.

Here are some extra tips from About.com:

• Always varnish the whole of the painting in one go. If you do only a part and this has started to dry before you do the rest, you'll end up with a line where the first bit ends.
• Try to have the same amount of varnish on the brush for each stroke so you put equal amounts of varnish on all parts of the painting.
• Work in a dust-free environment, otherwise dust particles will get stuck in the wet varnish. Keep cats out too; being so inquisitive, you could end up with paw prints in your new varnish.

And an extra-special tip on varnishing impasto paintings:

"... as well as brushing across the canvas in a sweeping motion, use the brush in a gentle stabbing action in the highly textured areas. You want to ensure the varnish gets into every nook and cranny, without puddling and without creating air bubbles or froth. Work systematically across the painting, and avoid going over areas where the varnish has started to dry as you risk making it cloudy."