Mix your media.
If you paint in acrylics, seal with Future or a compatible clear GLOSS paint. Make your wash with an organic solvent, mineral spirits, turpentin, Turpenoid, Humbrol, etc. It should be ‘mild’, not an aggressive material such as lacquer thinner. Apply your wash and wick it off with the same thinner.
If you paint in enamels seal with compatible clear GLOSS paint such as Model Master, Humbrol or your favorite. Make your wash from an acrylic solvent (water, alcohol) and use acrylic paints for the color. Apply and wick as before
Use a GLOSS sealer as it allows the wash material to be manipulated better Flats have pigment particles which trap the wash pigments resulting in a ‘cloudy‘ or stained appearance
CLear gloss also allows you to fix mistakes better. Happy with your paint job - seal it and try the wash. Not happy with the wash - remove it and try again. Happy with it - seal it and move to your nex step; a second wash, pastels, etc