I have always used a transparentizer from Grumbacher called GEL. Of course, since all major industries are experiencing shortages, why should painting supplies be any different? The original GEL made colors transparent but had no effect on consistency of the paint or drying time. The replacement product may be different. You will need to experiment until you find a product that suits your requirements. For now, I still have some of the original product. In this photo, I mix Titanium white, Flake white and gel. The amount of gel is equal to the amount of the two whites. I apply enough to cover all flesh areas. Then I use an old brush to work it into the canvas pores so that it is well covered. Then I use a strong but soft fan to slowly pick up the excess paint, wiping the brush frequently on paper towels. The grasaille or monotone painting will emerge. Then, using a soft fan or mop brush, I buff this to a high sheen.if you are not experienced and confident, I suggest allowing this to dry before continuing to refine the features.