How to Tell If Paint Has Gone Bad: A Painter's Guide

How to tell if paint is gone bad: the quick answer

Paint is usually still usable when it mixes smoothly, smells normal and spreads evenly; check each numbered acrylic pot for these signs before you begin.

• Open the pot and smell it briefly: a mild acrylic or plastic smell is common, while a strong sour, rancid or musty smell means the paint should be discarded.
• Look at the texture after stirring with a clean brush or cocktail stick; a little clear liquid on top is normal separation, while persistent lumps, strings, curdling or a rubbery mass indicate that the paint has deteriorated.
• Check for a skin across the surface, then lift it away if it comes off in one piece; small flakes can sometimes be removed, but a thick skin mixed throughout the pot will usually leave rough marks on the canvas.
• Stir for about 30 seconds and compare the colour with the number on the pot or the printed area; colour that stays streaky, grey, grainy or separated after mixing is a reason to stop using it.
• For a simple two-minute check, stir the pot, apply a small patch to spare paper or an unused edge of the canvas, and wait a few minutes to see whether it dries evenly without an unusual smell or gritty finish.
• Most pots in a paint-by-numbers kit remain fine when their lids have stayed closed, so test the colours you need first rather than replacing the whole set.

Why acrylic paint goes off in the first place

Acrylic paint goes off when water leaves the mixture and the binder begins to cure. In a paint-by-numbers pot, acrylic is a water-based emulsion containing pigment and a binder. The water keeps the mixture spreadable; the binder holds the pigment to the canvas after the paint is applied. Air reaches the paint each time the lid is opened, while warmth speeds up evaporation and curing.

Some changes are harmless. Pigment can settle at the bottom during storage, leaving thinner-looking liquid above it. This is normal and often improves with careful stirring. A settled pot may still produce smooth, even colour once mixed. Hard skin, rubbery pieces, a persistent sour smell or paint that stays lumpy after stirring point to a different problem.

Small numbered pots dry faster than large tins because they contain less paint in relation to the exposed surface around the lid. A little paint left on the rim can stop the lid closing tightly, allowing more air inside. Frequent opening during a project adds further exposure. This is common with adult paint-by-numbers kits, where many colours are opened repeatedly and some shades may remain unused for weeks.

When a pot can be saved and when it should go

A pot can usually be saved if the acrylic has separated or thickened, whereas strong smells, stubborn lumps and fully dried paint mean it should be discarded. Check each colour before it reaches the canvas, because uneven paint can leave gaps, raised patches or visible texture inside small numbered areas.

Storing your kit so the colours last

For a part-finished kit, careful storage helps the small acrylic pots stay usable between painting sessions.

• Wipe the rim clean, press each lid firmly into place and check that it has sealed all the way around before putting the pot away.
• Store closed pots upside down so the paint sits against the lid, which helps limit air exposure; place them on a flat shelf to reduce leaks.
• Keep the kit away from radiators, heated rooms and sunny windowsills, where warmth and direct light can make acrylic paint thicken more quickly.
• Avoid damp sheds, garages and other humid storage areas, as moisture can damage paper instructions, the canvas and the cardboard packaging.
• If you have mixed paint for a particular area, cover it between sessions with a clean, lightly damp cloth and use it promptly; keep the cloth from touching the painted surface.
• Write the colour number and the date opened on spare pots or their lids, then place them with the matching kit so you can identify older paint before starting a new section.
• Keep the canvas, paint pots, brushes and instructions together in a small box or tin with a secure lid, using suitable accessories to keep loose items organised.

What to do if a colour has gone and you still want to finish

If one colour has gone bad, you can often finish the canvas by checking where that colour is used and choosing the most practical substitute. Look over the numbered areas first. A colour may appear in only a few small sections, so you might decide to leave those areas until the rest of the painting is complete. This shows how much paint you actually need before you search for a replacement.

For a close match, mix small amounts from two usable pots on a spare piece of paper or card. Add one colour at a time, record the proportions, and let the sample dry before comparing it with the surrounding sections. Acrylic paint usually dries slightly darker, so judge the dry sample beside the canvas. Use the mixture only when it covers smoothly and the colour difference will be less noticeable in the finished picture.

If the missing shade is used widely, contact the shop to ask whether a replacement pot is available, or choose a fresh paint by numbers kit for adults if the remaining colours are also old. A slightly different shade can suit areas such as shadows, foliage or background texture, especially when it is applied neatly and consistently. For a cleaner finish, useful items such as a spare brush and mixing palette are available among paint by numbers accessories.

A note on unopened pots and personalised kits

Sealed paint pots usually keep far longer than opened ones because less air reaches the acrylic paint. Before starting an older kit, check that each lid is firmly closed and inspect the pots for leaks, swelling, cracked seals or dried paint around the rim. A sealed pot should still be tested on scrap paper before it reaches the canvas, especially if the kit has been stored for a long time.

Keep an unopened kit flat in a cool, dry place away from direct sunlight. Flat storage helps prevent paint from collecting around the lid, while sunlight and heat can affect the paint and printed canvas. Avoid sheds, conservatories and damp cupboards where temperatures can change sharply.

For a custom Paint by Numbers kit made from your photo, check every numbered pot when the parcel arrives. Look for damage, leaks or paint that has already dried, and contact the shop promptly if something is wrong. The 14-day right to cancel applies to eligible standard products but excludes personalised photo kits because they are made to your specifications.