Masking fluids work on both painted and unpainted surfaces. Note that some formulations of liquid mask may be unfriendly to acrylic paints. Humbrol Maskol and GSI Creos Mr MaskingSol Neo can and will attack Tamiya acrylics if applied too heavily. Also, Microscale's Micro Mask fluid shouldn't be used on a flat painted surface (gloss coat first before applying).
Remember, always try new techniques on a test hulk before working on a live project.