Is there a method by which fluidity can be restored to a bottle of Winsor and Newton Masking Fluid? My bottle, although several years old, is still more than three-quarters full but is now very tacky.

Winsor & Newton Technical Division reply: The ingredients in masking fluid are stabilised by ammonia and, if this should evaporate off over a period of time, you will end up with a rubbery mess in your bottle. If the addition of tap water does not immediately restore the masking fluid, unfortunately there is nothing to do by discard the bottle. To avoid the ammonia evaporating off in the first place, a bottle of masking fluid should be stored in a relatively cool place (but not freezing) and the cap kept secured. In addition to this, if you have already use a fair bit of the fluid, transfer it to a smaller container - the less air there is in the bottle, the longer it will keep.