How long does paint keep, and can I rescue a skinned or lumpy tin?

Part-used tins accumulate in every garage, and most of them are still perfectly good. Emulsion in a well-sealed tin kept above freezing keeps for years; the two things that finish it are frost, which breaks the emulsion irreversibly, and bacterial spoilage, which announces itself by smell. Solvent-based paints keep even longer but skin more readily.

Step by step

1. Smell it first, before you look at it.Open the tin and smell. Sound emulsion smells faintly of paint. Spoiled emulsion smells sour, cheesy or strongly of ammonia — that is bacterial breakdown, it is irreversible, and paint that smells like that will smell like that on the wall for weeks. Discard it.
2. Lift the skin off in one piece.A skin is simply paint that has cured in contact with air, and it is not a fault. Slide a knife round the edge and lift the whole skin out rather than stirring it in — stirred-in skin becomes the lumps you will be picking out of the finish all day.
3. Stir properly, from the bottom.Pigment and extender settle hard. Use a broad paddle or a mixer on a slow drill and work right down to the base of the tin until there is no dense layer left and the colour is completely uniform. Many a tin declared ruined has simply not been stirred.
4. Strain it before use.Pour through a paint strainer, a piece of old tights or a fine kitchen sieve into a clean container. This catches the lumps and dried rim material that would otherwise appear as nibs in the finish. On any tin that has been open before, this is worth the two minutes.
5. Test frost damage by looking at the texture.Emulsion that has been frozen and thawed can separate irreversibly into a grainy, curdled mixture that will not stir back together and feels like cottage cheese. If it does not become smooth after thorough mixing, it is finished.
6. Store what is left so it survives.Wipe the rim clean, tap the lid down flat all the way round, and store the tin upside down if it is sound — the paint seals the lid from the inside and no skin forms. Keep it somewhere frost-free and out of direct sun, which means indoors or a heated garage, not an unheated shed.
7. Decant small amounts into smaller containers.A litre of paint in a five-litre tin has a large air space above it and skins fast. Pour it into a jar or a small tin filled nearly to the top, label it with the colour, the finish and the room, and it will keep far longer.

Tips

• Write the room and the date on the tin with a marker when you finish a job. Two years later, that label is worth more than the paint.
• Boxing — pouring two or more tins of the same colour together and mixing them before you start — removes any batch-to-batch shade difference. Do it whenever a job needs more than one tin.
• A tin kept upside down genuinely does resist skinning, provided the lid is properly seated. Check it is not weeping before you put it on a shelf.

Common mistakes

• Stirring the skin back in — It does not redissolve. It breaks into rubbery fragments that end up as lumps in the film and clog the roller and the strainer.
• Storing paint in an unheated shed or garage — One hard frost separates water-based paint permanently. Store it indoors or somewhere that stays above freezing.
• Using paint that smells sour to save money — The smell transfers to the wall and lingers for weeks. It is a false economy that costs you the room.

If it doesn't work

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