Varnishing tips and techniques
Varnish protects paintings against mechanical damage (scratching) and air-pollution. The latter can cause pigmental discoloration. Secondly varnish can restore a painting's original colors.
Modern varnish is usually a synthetic resin. As it dries, a chemical process takes place in which the resin releases a vapour which is harmful to your health, so you should provide good ventilation to your workplace.
When paint dries it's color loses some of it's brilliance, which is especially the case with acrylics, but oils too become somewhat darker. This can be annoying if the painting is unfinished because a newly applied color which looks identical to an already applied color will become dissimilar when it dries. To avoid this, retouching varnish can be applied to paint which is "hand-dry". Retouching varnish differs from regular varnish in that the latter can be applied only when the paint is dry through and through. When applied before that, the painting will get "craquelured" because varnish dries quicker than oilpaint. Also, when applied thickly, the paint will even start to rot, I'm not kidding. When applied thickly wait for a year before you apply the permanent varnish.
One tricky problem is dust. Particularly if you paint slowly your painting will pick up dust which is clearly visible on the dark areas of your creation. Some people recommend using wet cloth with detergent for getting the surface dust-free. The problem with that is, that a wet cloth is somewhat adhesive and rough and can damage the paint, breaking off extrusions. I like to use dry micro-fiber cloth with which you can wipe the dust off. This is quite effective and low risk.
Painting and varnishing tips
One tip I'd like to pass on to you is to spray-apply your varnish. Not only does it look better (you'll get a very thin and smooth layer), but it's quick and easy and it's particularly recommended if you use Winsor and Newton's Oil Bar. This medium has it's own varnish but it can only be applied a year after the paint is hand dry. Before that, the varnish will actually dissolve the paint, when applied with a brush. This can be avoided if spray-applied, because the very thin layer of varnish will only dissolve a very thin layer of oil bar, which will stay in it's original position because it's not smeared out with a brush. It's not perfect, because rough areas (which have more surface area per squared inch of canvas) will require more varnish, which is hard to do. If you're not convinced that every spot is well varnished, then you must apply a second layer over the whole painting. Don't try to "fix" local areas because you'll end up with an unevenly varnished painting.
Faq's about varnishing oil paintings
How thick should I apply the varnish?
Thin! A thinly varnished painting will look better and more professional.
If I have applied retouching varnish, will I still need to apply a permanent varnish?
Yes. Retouching varnish doesn't have the quality that a finishing varnish has. A finishing varnish makes your painting look better.