About 20% of white dwarfs possess a magnetic field that may be detected by the splitting and/or polarisation of their spectral lines. As they cool, white dwarfs evolve to such a low effective temperature that no spectral lines can be seen in the visible wavelength range. If their atmosphere is not polluted by the debris of a planetary system, such cool white dwarfs have featureless optical spectra. Until quite recently, very little was known about the incidence of magnetic fields in such objects. However, when observed with polarimetric techniques, a significant number of featureless white dwarfs reveal the presence of a strong magnetic field in their optical continuum spectra. Measuring the occurrence rate and strength of magnetic fields in old white dwarfs may help to understand how these fields are generated and evolve. Here we report the results of an ongoing survey of cool white dwarfs with the high precision broad-band polarimeter DIPOL-UF deployed at the Nordic Optical Telescope, La Palma, Spain. This survey has so far led to the firm discovery of 13 cool magnetic white dwarfs in the solar neighborhood, including six new detections reported in this paper.