Fast - Hard 25 Si. Fast - Heavy 20 and Slow - Heavy 20 B72 Retouching Gels are recommended for painting with a palette knife. Fast drying gels should be used for painting thick layers and high impastos. Paint with a slow drying gel can be used for extensive reconstructions and should be applied in thinner layers.
The same principles apply to painting with a palette knife or spatula as apply to painting with impastos. Paint for heavy impastos and textures should be rich in pigment in order to avoid shrinkage and subsequent cracking during the drying process. Light 10 and Very Light 7.5 gels should not be used for painting with a palette knife. Having a false body made mainly of solvent they are comparable with the traditional megilp. The resulting thick paint layer and textures will crack and be brittle.