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Lara,
It is best to use an easily removable varnish designed for oil paintings. The best seem to be the so-called acrylic resin varishes. These synthetic varnishes do not yellow over time like the traditional natural resin varnishes (mastic and dammar) and are easier to remove too. You can buy some of them in spray cans.
Painting varnishes are traditionally applied with the brush. I prefer to use Gamblin's Gamvar varnish which I brush on. A painting should be allowed to dry for at least three months, better if allowed to dry six months or a year. If a painting is varnished too soon, it will eventually "eat" the varnish (asborb the varnish into the paint film) as it continues to oxidate and parts of the painting will become dull again.
I live in a very dry climate and I varnish this way:
First -- I wet an old washcloth and wring it out as dry as possible. I wipe the surface of the painting with this cloth.
Second -- I examine every square inch of the canvas for lint or cat hair or whatever... I wipe the surface with the dry palm of my hand to pick up anything I see. My hand leaves no lint and I'm not a werewolf, so I noticed no hair on the palm of my hand the last time I looked.
Third -- I lay the painting flat and apply the Gamvar with a soft flat synthetic hair brush. I stroke it out as far as possible. I try and apply an extremely thin coat only. Gamvar and other synthetic varnishes are much easier to apply than traditional dammar and also much easier to remove later.
Fourth -- After only about five minutes, the varnished painting can be placed in the vertical position without fear of the varnish running. I lean it against a wall, the paint side facing in. This prevents dust settling on the varnish as it dries. In my climate, the varnish is dry in twenty minutes or so.
A varnish serves to give the finished painting an even sheen and to protect the paint from the smoke and dirt in the atmosphere.
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