Acylic paintings, just like oil, need to have a synthetic removable varnish applied after they dry. Oil paintings generally need 6-12 months to dry, acrylics dry much quicker and should dry completely anywhere from 1 to 7 days.
As Wally, said the varnishes come either brush on or in an aerosol spray. Various art material manufacturers supply these. I generally use Winsor Newton, both brush on and spray.
The best approach is to brush on the first coat, let it dry (about an hour) and then several light coats of spray working one direction one time and at 90 degrees for the next coat.
There are two purposes for the varnish:
1.) Cosmetic: The varnish will give a uniform sheen or matte surface depending on the varnish selected. Acrylic paints mostly have a matte finish when they dry. Oil paints vary in their degree of sheen depending on the color used. A final varnish will even out the sheen to the desired degree of glossiness. I generally find a semi-gloss to be the most attractive varnish.
2.) Protection: The only way to effectively clean any painting is to remove the final varnish from the painting thus removing the dirt and grime accumulation from atmospheric polutants such as smoke. If a painting has not been properly varnish, these polutants will impregnate themselves into the top layer of paint on a painting and are very difficult, if not impossible, to remove without taking away a portion of the top layer of paint, thus destroying the painting.
Years ago, oil paintings were painted with a final coat of damar varnish. Damar varnish is more difficult to remove than the new sythetic varnishes and tends to yellow over time. Synthetics are easily removed with patient care using mineral spirits, Acetone, Toulene and other substances and combinations of them.
Dave Makielski