From Dave Bernard, Gamblin Colors:


Adding CWM to Gamvar will reduce the sheen of Gamvar Gloss.  Before we ever made Gamvar Satin and Gamvar Matte, adding a percentage of Cold Wax (slurry) was the best way to make your own satin version.

 

Here are the specifics on gloss reduction out of our notes on using Gamvar Gloss:

 

Gloss Reduction Techniques:

  1. Dilute Gamvar with up to 25% Gamsol.  A thinner coating will be slightly less glossy.
  2. Brush the varnish as it dries.  When the varnish is starting to set up, continuing to brush it will microscopically roughen up the surface resulting in slightly less gloss.
  3. Add 10-30% thinned Cold Wax Medium to Gamvar. 
    1. Use a glass or clear plastic jar with a screw top lid for making the mixture, preferably something with a wide enough mouth to accommodate the width of your varnish brush.
    2. To make a 20% wax mixture, mark the side of the jar with 5 equally spaced lines for gauging proportions. Fill up to the 4th line with Gamvar Gloss.
    3. Scoop out enough Cold Wax Medium to accommodate the remaining 20% volume and place on a clean palette.
    4. Add Gamsol a little at a time to the wax and mix thoroughly with a palette knife until you achieve a soft, slurry consistency.  This is your matting agent.  (Save any leftover in a separate jar for future varnish and painting medium gloss reduction.) 
    5. Add the wax slurry into the mixing jar until the varnish level reaches the 5th line. Cap and shake the mixture intermittently over 20-30 minutes until the varnish, held up to the light, looks uniformly cloudy with no wax floaters. 
    6. Test the mixture on the corner of your painting and allow it to dry to judge the results. If you need it glossier, simply add more Gamvar.  If you need it more matte, follow the previous steps to add more Cold Wax slurry. Once you are satisfied with the surface, continue to varnish the rest of the painting.
  4. Rub on a thin coat of Cold Wax Medium with a soft, lint-free cloth using small circular motions.  (TIP: Gently warm the wax container (lid on) in a pan of hot tap water to soften it for easier spreading.  Apply a very small amount of wax to the cloth- too much wax on the cloth will be difficult to rub out uniformly.)  Allow the wax to dry completely overnight for a matte surface.  For a satin sheen, allow the wax to dry 4 – 6 hours and lightly buff with a soft, lint-free cloth.