Have you tried Atelier Interactive Acrylics - these are quite revolutionary. Lets say your painting begins to dry you can make it wet again just by spraying a fine mist of water into the area you wish to re-work.
This can only be done if your painting has been left for an hour or so, if you your painting has been left over a weekend for example you will need to spray on something other than water - unlocking formula. This will have the same effect and is available with the Atelier range.
The beauty of Atelier Interactive is that it dries from the inside out making its drying time slightly longer than conventional acrylics.
Don’t forget to check out http://www.saa.co.uk/artmaterials/atelier-interactive-acrylic-paint-sets-2405.html