I did some research on misted dials and it seems that the combination of older technology (hot wire etc) and incandescent lighting causes the condensation due to the heat inside the gauge.
The modern solution is to use low power electronics and led lighting.
Modifying older gauges to eliminate condensation is difficult, with various schemes such as drilling vent holes, inner glass treatments, hermetically sealing and drying the air with a desiccant, filling with an inert gas all having various degrees of success
I have one gauge which misted frequently so I first tried drilling an extra air vent (fail) and then tried to hermetically seal the gauge (fail).
In the end I tried (as an experiment) using a small 12V aquatic air pump to gently blow air through the gauge. This actually seemed to work.
If the misting comes back this winter I will probably install the pump as a longer term test.
