Sand the surface of the pink tile with your vibrating sander and medium sandpaper, dulling the shine. Wipe off the dust.
Apply oil primer over the tiles with a paintbrush in a thin, smooth coat, with the brush strokes going all in one direction. Don’t paint the grout lines.
Let the primer set for 24 hours.
Brush oil paint onto the tiles in a thin, smooth coat, covering just the tile face and not the grout lines. Don’t worry that the paint isn’t completely covering the primer, as multiple thin coats are preferable to fewer thick coats. Keep the brush strokes going all in one direction.
Let the paint set for 24 hours. Add two more coats in the same manner, letting each of them dry for 24 hours.
Brush on a layer of oil-based polyurethane in the same manner as you did the paint, in a thin, even coat that covers just the tile surface. Let the polyurethane set for 24 hours.
Lightly buff the polyurethane by hand with extra-fine sandpaper, to dull the gloss. Brush off the dust.
Apply a second polyurethane coat. Let it dry for 24 hours. Dull it with extra-fine sandpaper and brush on a third coat. Let it set 24 hours.