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MITRA Forum Question Details

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  • use of 4F pumice to create tooth in acrylic primersApproveRejectUn-ApproveSubscribeUn-Unsubscribe
    Question asked 2017-10-10 11:04:40 ... Most recent comment 2017-10-10 18:26:00

    ​I've heard that  a little 4F pumice, added to  acrylic primer, creates a smooth oil painting surface on rigid panels but with a little more tooth to grab onto the paint.   Acrylic primer alone seems a little slick for me, but I do not necessarily need great absorbancy.  

    Any problems?

    Would this surface be too abrasive for bristle or sable brushes?

    Would the pumice increase absobancy? 

    Thanks for your thoughts.

Answers and Comments

  • ApproveRejectUn-ApproveUser Comment

    ​Would marble dust be preferrable to pumice or rottenstone to create tooth in an acrylic primed panel??

    2017-10-10 12:48:12
  • EditDeleteModerator Answer

    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!

    Matthew Kinsey, Utrecht Art Supplies
    2017-10-10 16:17:27
  • ApproveRejectUn-ApproveUser Comment

    ​Does "acrylic pumice" mean that there is stone pumice in an arcylic medium, or that the grit is made from acrylics and placed into an acrylic medium?

    Assuming that the acrylic pumice in the medium is different from ground pumice, is it also hard on brushes?

    If the acrylic pumice is hard on brushes, would I be better off with the fine or coarser acrylic pumice medium to minimize excess brush wear?

    Would rottenstone work any better to minimize brush wear?  I understand that it is finer than the pumices.  Both can be purchased in woodworking stores.

    All I need is a little more tooth in the painting surface so that the paint comes off the brush better.

    Thanks for your thoughts.

    2017-10-10 18:10:57
  • EditDeleteModerator Answer

    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. 

    Matthew Kinsey, Utrecht Art Supplies
    2017-10-10 18:26:00

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