  We report an experimental study on the drying of silica colloidal
dispersions. Here we focus on a surface instability occurring in a drying paste
phase before crack formation which affects the final film quality. Observations
at macroscopic and microscopic scales reveal the occurrence of the instability,
and the morphology of the film surface. Furthermore, we show that the addition
of adsorbing polymers on silica particles can be used to suppress the
instability under particular conditions of molecular weight and concentration.
We relate this suppression to the increase of the paste elastic modulus.
