The spacing depends upon the thickness of the paint, how viscous it is and perhaps also upon the roughness of the wall to which the paint is applied. If one liquid is moved through another having different characteristics, this same development of fingering occurs. The same is true of two gases moving against each other, as seen in the pattern of smoke curling up from a cigarette. This curious tendency for moving liquids and gases to develop filamentary structure is known as the Rayleigh-Taylor instability. It can be described by mathematical formulas which clearly indicate that the fundamental underlying cause is the difference in viscosity of the materials involved.
Viscosity tells how well a material clings to itself.
The greater the difference in viscosity between two gases or liquids thrust together, the greater is the spacing between the filaments that appear. And even rocks, which we normally consider to be solids, can flow like liquids over a long period of time. Thought of this way, it seems quite natural that the volcanic magmas would undergo the Rayleigh-Taylor instability and so create an evenly spaced row of volcanoes.