Why does paint come off with spirits but not with water?

Answer 1

Good question. Some materials are more soluble in one solution than another.

In general (very general); non-polar things are soluble in non-polar solutions.

Polar things are soluble in polar solutions.

So - for example - salt is NaCl, which is Na+ Cl- and so very polar. It dissolves well in water because water is also polar. (Water is kinda like H+ O- H+ if you excuse the poor charge balancing for the sake of simplicity).

Paint however may be (not always, you can get water soluble paints) very non-polar and so you need something non-polar to dissolve it. White spirit is made from aliphatic (and alicyclic) compounds...which means hydrocarbons. These are not polar since C and H are actually pretty close in terms of electronegativity and they have no overall charge.

And so when you put white spirit on the paint, the paint dissolves into the white spirit and so the paint "comes off".

For further reading try: http://en.wikipedia.org/wiki/Solubility

For more advanced understandings, perhaps start looking into thermodynamics and the hydrophobic effect as they pair up nicely and kind of help each other in terms of understanding.