Its not unusual to see a watercolour painter using a hair dryer to speed up the drying of background washes before the final details are added on top. This ensures that the details are sharp and not blended into the wet surface. The initial dark colours in oil painting are sometimes mixed with a solvent only which will evaporate quickly and not interact too much with the final colours. In this painting I needed to evaporate the solvent quickly as I wanted to finish the painting in about 1 hour. So I used the hair dryer. The downside of this is that these colours will become lighter in colour and affect the apparent tonal range within the painting. You must ignore these parts and press on with the painting. If you paint a picture over several sessions allowing the previous layers to dry the usual practise is to ‘wet out’ the painting with something like ‘Liquin’ and the painting looks as if it is freshly painted without the problems associated with painting into wet paint. For example, suppose you are painting a landscape which has a tree in front of a sky. You have painted the sky which is now dry and you you now want to paint the tree. You ‘wet’ the sky with a thin layer of ‘Liquin’. As you are putting in the details of the tree the lighter colours of the sky are not contaminating the darker colours of the tree. It feels like you are painting onto wet paint with the advantage that if you need to make a correction the offending piece can be removed easily with a tissue paper dampened with solvent. It allows you to have several attempts at painting the tree as in our imaginary landscape above.
Check out the video in previous post.