ShelfYears

Does paint thinner go bad?

Klean-Strip says its paint thinner has no expiration date in a cool, dry spot — then puts two real numbers under that: more than 3 years unopened, and 6 months once the cap has been off.

Updated

Paint thinner
Sealed
3+ years
Opened
6 months
EVAPORATION & CONTAMINATION
Degraded performance

Sealed: 3+ years Tier 1

The maker's own wording, repeated on both its paint thinner and its odorless mineral spirits pages: the product "has no expiration date when stored in a cool, dry location," and "the shelf life is greater than 3 years unopened and 6 months opened when stored in a cool, dry location and protected from external damage to the packaging." A sealed, undamaged can of mineral spirits is close to inert.

After opening: 6 months Tier 1

The opened figure is 6 months and the reason is the cap. Every opening lets the lightest fraction of the solvent go and lets room air, with its moisture and its dust, take that place. A can that has had brushes rinsed in it is carrying whatever came off them — pigment, resin, water — and it will carry that into the next thing it touches.

How to tell it's gone off

  • Cloudiness where fresh thinner is water-clear.
  • A colored tint, or a settled layer of pigment or resin sitting on the bottom.
  • A separate water layer, or beads of water clinging to the inside of the can.
  • A sour or sharpened smell from the container instead of the ordinary petroleum smell.
  • A can that has been frozen, dented or rusted — Klean-Strip's shelf life assumes packaging that has not been damaged, and asks that the product be kept from freezing.

If you use it anyway

Degraded performance

Old thinner from a sealed can is a non-issue; old thinner from a can that has been open for years is a weaker, dirtier solvent than the one on the label. It cleans more slowly, it leaves what it was carrying behind on the surface, and thinned into a finish it takes its contamination with it. The realistic cost is a job that has to be done twice — but check the can against the light first, because a cloudy one is telling you what it has been holding.

Why it goes off — EVAPORATION & CONTAMINATION

Mineral spirits is a stable hydrocarbon with no additive package to spend and no oxidation clock worth the name — sealed and cool it barely moves in 3 years. Both things that change it follow from the container being opened. The most volatile part evaporates, so what remains is slightly heavier and slower than the label describes, and anything that has been rinsed in the can stays in the can once the solvent it was suspended in has gone.

Every page here commits to one of four states — Harmless · Degraded performance · Equipment damage · Safety issue. How the verdicts work.

How to store it

Klean-Strip's three lines, in its own order: keep the product from freezing, keep the container tightly closed when not in use, and store it in a cool, dry place. The company adds a fourth condition that is easy to miss — its shelf life figures assume packaging that has not been damaged, so a dented or rusting can is outside the number whatever the date says.

Do not pour this down a drain.

Sources