Varnishing a gouache painting should be avoided, because the varnish drastically affects the depth, darkness and finish of the work. And should anyone want to retouch the painting again at some point in the future, perhaps for restoration purposes, the varnish will not come off.
Your technique may have led to what is known as dusting off, where the surface of the painting looks dusty, but once again varnishing will not help. Dusting off occurs when the colour has been diluted with too much water, leaving only pigment on the surface and an underbound painting. The solution is to avoid this happening in the first place and to add gum arabic when preparing your colours.
But this does not mean gouache paintings need to be left unprotected. Framing the work behind glass will help. Remember to put a small distance between the glass and the work itself – paintings placed directly against glass are prone to condensation and the growth of mould. A mount between the paper and glass allows just enough circulation to prevent this.
To help your work last over time, consider the permanence ratings of the colours you are using. Here’s how the system works:
- AA and A: colours with an AA or A rating, such as Brilliant Yellow and Sky Blue, are not expected to fade.
- B and C: colours with a B or C rating, such as Light Purple (B) and Parma Violet (C), tend to fade over a 10-year period.
- Fugitive: some colours within the C band are known as fugitive, which means transient. These may fade within months.