This article is from the Model Rockets FAQ, by Wolfram von Kiparski with numerous contributions by others.
Most competition models are not painted in a normal
sense. Many gliders are left totally unpainted. Some modelers
will color the model with magic marker or a thin layer of
model airplane dope for visibility. Others will apply a coat
or two of clear dope to prevent warping. I personally prefer
Japanese Tissue and dope (discussed later), as it adds both
strength and color to the model, at a very minimal weight
Conventional finishing techniques of filler, primer, paint, and decals
should be left to models where glide performance is not a concern.