This study presents a new approach to enhance the corrosion resistance of tungsten inert gas (TIG) welded AZ31 magnesium alloys by using nanocrystalline magnesium fluoride suspensions in a suspension plasma spray (SPS) process. We have developed a synthesis for the preparation of nanocrystalline magnesium fluoride suspensions, which delivers nearly monodisperse nanoparticles in a gram scale yield. The particles were analyzed with transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD). Stable suspensions of magnesium fluoride nanoparticles in water were characterized by dynamic light scattering (DLS), zeta-potential, and viscosity measurements. Such suspensions were deposited with an SPS torch onto TIG welded seams of the magnesium alloy AZ31, thus producing a protective magnesium fluoride layer. Magnesium fluoride covered welded seams were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDXS). In order to introduce a simple method for sensoring the deposited magnesium fluoride coatings, the magnesium fluoride nanoparticles can also be fluorescence-labeled by co-doping with cerium(III) and terbium(III), the respective optical properties were characterized by reflection and luminescence spectroscopy. The deposited layers can, thus, be inspected by illumination with an UV lamp, because of their bright green emission. The corrosion properties of the magnesium fluoride layer on the welded seams were studied by means of potentiodynamic potential measurements.