We have studied the stability of ink-jet printed lines of liquid with zero receding contact angle on a homogeneous substrate. Such lines can become unstable by forming a series of liquid bulges, at various wavelengths, connected by a ridge of liquid. The instability was studied with a simple dynamic model. It was shown that the line becomes unstable when the contact angle of the liquid with the substrate is larger than the advancing contact angle. This condition, however, is not a sufficient condition. When the transported flow rate is sufficiently small compared to the applied flow rate a printed line can be shown to be stable, i.e. the width of the printed line is constant. This was found to depend on the advancing contact angle of the liquid over the substrate.