You finish a canvas, frame it, hang it, and some weeks later there are specks of color on the inside of the glass. The paint felt dry when you framed it. It was dry to the touch. It was not cured, and those are different things.
This is one of the most avoidable problems in the whole hobby, and almost all of the fix happens before the frame goes on.
Why it happens
Acrylic dries in two stages. The water leaves in under an hour, which is what makes it touch-dry. The film then goes on knitting together for days or weeks afterwards, and during that period it stays slightly soft.
Worse, an acrylic film never gets truly hard the way an oil film does. Even a fully cured acrylic surface remains a little tacky at a microscopic level. Press it against a smooth sheet of glass, leave it there, add some warmth, and it will bond to the glass. When the frame is opened, the paint comes away with it.
Four things make it likely:
- Framing too soon. The main cause by a distance.
- Direct contact between paint and glass. The second cause, and the one that persists even years later.
- Thick applications. Heavy ridges of paint dry slowly right through, and they are also the high points that touch the glass first.
- Heat and humidity. A frame above a radiator, in a conservatory, or on a wall that gets afternoon sun goes through repeated warm cycles, and warmth softens the film again.
Preventing it
Wait longer than you think
For a paint by numbers canvas painted in normal thicknesses, give it at least a week in a dry, ventilated room before framing. Two weeks or more if any area is thickly applied. It is a dull instruction and it prevents most of what follows.
Leave it flat and uncovered while it waits, out of direct sunlight, with nothing resting on it. Handle it by the edges.
Do not let paint touch glass
This is the structural fix, and it is what professional framers do as a matter of course.
Frame spacers are thin strips that sit inside the frame between the glass and the artwork, holding a small gap. A mount board or mat does the same job — the window sits over the artwork and the board itself keeps the glass off the surface. Either is enough, and the gap also lets air move, which matters as much as the separation.
The simplest option of all is not to use glass. Acrylic paintings on canvas are conventionally displayed unglazed, and a completed paint by numbers canvas hangs perfectly well in a frame with no glazing at all. That removes the entire problem.
Consider a varnish
A picture varnish gives the surface a harder, less tacky skin, and it means any dust or contact lands on the varnish rather than the paint. It also evens out the patchwork of sheens you get on a canvas painted section by section over weeks, which is worth having on its own.
Wait the same week or more before varnishing, work flat, and use a satin finish unless the design is very dark. Our note on varnishing an acrylic painting covers the choices and the isolation coat that goes underneath a removable varnish.
If it has already happened
Take the piece out of the frame first so you can see what transferred where. Then treat the glass and the painting as two completely separate jobs, because what is safe on one will destroy the other.
On the glass
Lay the glass flat on a towel. Warm water with a little dish soap on a soft cloth will lift most transferred acrylic with gentle pressure and patience. For anything stubborn, isopropyl alcohol on a cotton bud works well — acrylic dissolves in it readily, which is exactly why it is the right tool here.
Do not scrape with a blade or use an abrasive pad. Use a microfiber cloth rather than paper towel, which sheds fibers and can scratch acrylic glazing.
On the painting
Never use alcohol or any solvent on the painting itself. This is the one instruction on this page that matters most. Acrylic paint dissolves in alcohol, so a swab that cleans the glass beautifully will take your painting off the canvas. The same applies to glass cleaner, nail polish remover, white spirit and every other solvent.
Where paint has lifted, the repair is paint. Wait until the surface is completely dry, then match the color from the kit's own pots and reapply thinly to the bare patch, matching the thickness of the surrounding paint rather than filling the gap in one heavy stroke. Two thin coats blend in better than one thick one. Then leave it another week before it goes near a frame again.
If a section came away and you no longer have that color, the matched extra paint set for the design has the same pots as the original kit.
Where to hang it
Once the piece is back together, the wall does more for its survival than anything else. Keep it off exterior walls that get damp, away from radiators and heating vents, out of a bathroom or a kitchen where it will take steam and cooking grease, and out of direct sunlight. Somewhere that stays at a steady, ordinary room temperature is all it needs.
If you are storing framed pieces rather than hanging them, stand them upright rather than stacking them flat, with something soft between each one, in a cool dry place. Weight on a stack is pressure on the paint.
The simpler route
Most of this problem exists because of glass. A framed paint by numbers canvas — stretched on its wooden frame, varnished or not, hung without glazing — has nothing to transfer to. Our framed kits arrive already stretched for that reason, which also means there is no mounting job at the end. You can see them in the 12 by 16 framed range and the full collection.
If you have painted an unframed canvas and are deciding how to mount it, our guide to framing a paint by number canvas goes through the options, including why stretching should happen before varnishing rather than after.
