You do not have to varnish an acrylic painting. Plenty of finished work hangs unvarnished for decades and is perfectly fine. But there are three specific reasons to do it, and one property of acrylic that makes the decision less obvious than it is with oil.
Why varnish at all
Because dried acrylic stays slightly soft. This is the property that matters. An acrylic film never fully hardens the way an oil film does; it stays a touch tacky at a microscopic level, and it attracts and holds dust and airborne grease. On an unvarnished canvas that dirt is embedded in the paint itself and cannot be cleaned off without taking paint with it. A varnish layer gives the dirt somewhere else to land.
Because it evens out the sheen. Different pigments dry to different finishes. Some areas come up satin, others matte, and on a canvas painted in sections you can see the patchwork under raking light. This is particularly visible on a paint by numbers canvas, where every region was painted separately and some got two coats and some got one. A single varnish coat unifies all of it at once, and this is the reason most worth having.
Because it deepens the darks. A gloss or satin varnish restores the saturation that matte drying takes out. Dark colors especially look flat and slightly chalky when acrylic dries, and a varnish brings them back to what they looked like wet.
Why you might not
If you may still work on the painting, do not varnish it. Painting over varnish does not adhere properly, and removing varnish to get back to the paint is a job with real risk attached.
If the piece is practice, or a canvas you expect to paint over, skip it. And if you simply want to hang the thing and get on with the next one, hanging it out of direct sunlight and away from a kitchen or a fireplace will get you most of the way there.
How long to wait
Longer than most guides say. Acrylic is touch-dry in under an hour, but touch-dry is not cured. The water leaves quickly; the film goes on knitting together for days afterwards.
For a paint by numbers canvas painted in normal thicknesses, wait at least a week. For anything with thick or heavily built-up areas, two weeks or more. If you varnish over paint that is still curing you can trap moisture underneath, which shows up later as a cloudy bloom you cannot remove.
Do not test dryness by pressing a fingernail into the paint. You will leave a mark in a film that is still soft. Look at it under an angled light instead — paint that is still curing often has a slightly different sheen from the areas around it.
The isolation coat, which most guides skip
This is the step that separates a varnish job you can undo from one you cannot, and it is the reason so many people end up with a painting they dare not touch.
A removable varnish is designed to be taken off later with solvent, so a conservator can clean the surface and revarnish. The problem is that acrylic paint is itself soluble in the same solvents, at least partly. Take the varnish off an unprotected acrylic painting and you risk lifting color with it.
An isolation coat solves that. It is a thin layer of clear acrylic gloss medium, brushed or sprayed over the finished painting and left to dry, sitting between the paint and the varnish. It is permanent, it is not soluble in the solvents used to remove varnish, and it seals the surface so the varnish sits on top rather than sinking into porous areas.
Two coats of gloss medium thinned slightly with water, brushed in one direction and left to dry between coats, is the usual approach. Use gloss even if you want a matte finish — the matting agent goes in the varnish, not the isolation coat, and a matte isolation layer will cloud the painting underneath.
For a paint by numbers canvas that is going on a wall in a family home, this is optional. For anything you would be upset to lose, it is worth the extra evening.
Gloss, satin or matte
This is entirely a matter of what the painting is and where it hangs.
- Gloss gives the deepest darks and the strongest color, and it also reflects every light in the room. It suits dark, dramatic subjects — night scenes, water, anything where the shadows are doing the work.
- Satin is the safe middle. Most of the saturation, far less glare. If you are unsure, choose this one.
- Matte removes reflections completely, which is useful opposite a window, but it also flattens dark areas and can leave a faint haze over deep colors. It is the least forgiving choice on a dark painting.
Whichever you pick, stir it thoroughly rather than shaking it. Matting agents settle to the bottom of the tin, and shaking puts bubbles in a layer that has to dry clear.
Water-based or solvent-based
Water-based acrylic varnishes are what most people should use. They clean up with soap and water, they have almost no smell, and they go on with an ordinary brush. The one thing to know is that they are milky white in the tin and dry clear, which is unnerving the first time and makes it hard to see where you have already been. Work in good raking light so you can see the wet edge.
Solvent-based varnishes go on clear, level out more smoothly, and are what conservators generally prefer. They also need ventilation and mineral spirits for cleanup, and the fumes are not pleasant in a small room. Unless you have a specific reason, the water-based version is the better trade.
How to apply it
- Clean the surface. A soft dry brush or a lint-free cloth. Any dust you leave is under the varnish permanently.
- Lay the canvas flat. Varnish on a vertical surface runs, and a run dries as a visible ridge.
- Work in a warm, dry room. Above about 60 degrees Fahrenheit, and not on a humid day. Cold or damp air leaves acrylic varnish cloudy.
- Use a wide soft brush — two or three inches — and load it properly. A dry brush drags and leaves streaks.
- Work in one direction across the whole canvas in overlapping strokes, keeping a wet edge and not going back over an area that has started to set. If you want a second coat, let the first dry fully and lay it at right angles to the first.
- Do not flood it. Excess varnish pools in the low points and dries cloudy there. Thin and even beats thick and generous.
- Leave it alone while it dries. Cover the room from dust if you can, and do not touch it to check.
What UV varnish does and does not do
Some varnishes contain ultraviolet light stabilizers. They absorb part of the UV that reaches the surface rather than reflecting it, and they genuinely slow fading — but they slow it, they do not stop it, and the protection is partial.
Nothing in a tin will save a painting hung in direct sunlight. If a canvas matters to you, the wall it goes on matters more than the varnish on it. Out of direct sun, off an exterior wall that gets damp, and away from a kitchen or an open fire will do more for its life than any coating.
For a paint by numbers canvas specifically
The evening-out of sheen is the real prize here, more than protection. Our kits are painted section by section over hours or weeks, and the finished surface almost always has a patchwork of finishes across it. A single satin coat pulls it together and makes the canvas look like one painting rather than a jigsaw.
Wait a week at minimum, work flat, use satin unless the design is very dark, and one coat is usually enough. If you have painted an unframed canvas and plan to stretch it, do the stretching first and varnish afterwards — varnish on a canvas that is later pulled around a frame can crack along the fold. Our guide to framing a paint by number canvas covers that order of work.
Whatever you use, wear gloves and keep the room ventilated. Our note on what is actually hazardous in paint covers the sensible precautions, and you can see the designs themselves in the full collection.
