Thank you Joe, Jade, and Donna.
I have given up on the paint retarder and marble dust. I now use a medium of Matt gel and extra heavy medium. It softens the pant and gives a nice satin finish which I have come to like. I use mostly hog hair Bristol flats and a couple of round synthetic hair brushes. I have gotten to a point where I paint fast enough to get through a painting without having o deal with dried paint on the palette. I use a 16 x 20 glass palette so I have room to get away from dry spots. I put my leftovers on parchment paper in an air tight container from the dollar store and add a little water dampened cloth in one end of the container. Keeps things nice and soft and useable for days.