• Question: Why does paint dry on the walls but not in the tin?

    Asked by tillieissocoollike to Carla, Madgie, Nick, Vicky, Werner on 13 Nov 2013.
    • Photo: Marlene Lorgen

      Marlene Lorgen answered on 13 Nov 2013:

      I love this question, but had to look up the answer as I had no idea! So most paints are water based, and the rate of water evaporation depends upon the vapour pressure above the surface of a liquid. In a sealed paint time, vapour pressure can build up in the tin above the paint, meaning that the water in the paint will not evaporate, keeping it liquid. When the paint is put on the wall, the water can easily evaporate, leaving a dried film of solid paint on the wall. In oil based paints, oil does not evaporate, but undergoes a chemical reaction when exposed to air, causing the paint to set.

    • Photo: Nick Groves-Kirkby

      Nick Groves-Kirkby answered on 13 Nov 2013:

      I think Marlene has this one covered!

      I’ll just add that the usual reason we put things in tins is to keep them sealed from the outside world. Paint tins keep moisture in and prevent paint from drying out, and baked bean tins prevent microbes from getting to your food and it going rotten. You might also keep tobacco or firelighters in a tin to keep them dry (rather than to keep them wet like paint).

    • Photo: Vicky Forster

      Vicky Forster answered on 13 Nov 2013:

      I don’t know the answer to this one, but just popped in to say, great question and Marlene’s answer is very interesting. I didn’t know that.

    • Photo: Werner Muller

      Werner Muller answered on 14 Nov 2013:

      Marlene is the perfect painter.

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