MITRA Forum Question Details

It's tricky to add marbledust to factory-prepared acrylic dispersion primer (gesso) because it tends to yield a weak, crumbly film when added in excess. Pumice is less absorbent (it's more like glass) and gives a good mechanical texture which facilitates good paint attachment. Acrylic pumice medium or Garnet Medium can be combined with acrylic gesso to add texture without taking up too much moisture from the primer.

Braque is known to have added sand and other textural inclusions in his grounds, which were oil based. I don't think it's been established whether the "sand" was pumice, but pumice was present in his studio, so I think it's likely that he did use it on the pictures. Pumice is brutal on brushes, though!

Yes, Acrylic Pumice Medium is a factory-prepared product made with fine pumice in acrylic dispersion medium. There are other products like this, including Garnet Medium. These can be added to acrylic colors and primers without having to deal with unbound particles or crumbly paint.

Acrylic Gesso alone is somewhat abrasive, and can wear down brushes. Garnet and pumice are both pretty rough on bristles, even in a medium. Brush wear can be minimized by using enough paint so that brushes move easily, but I would not use nice sable brushes with any aggressively textured medium

Rottenstone, like marbledust, can affect film strength when used ​in excess, and might yield a ground that is too "thirsty". If you do experiment with these, you could perform a crude test for flexibility by painting samples of modified gesso on mylar, and flex the dry samples to determine whether cracking will be an issue.