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I need to do an unusual molding of an outer corner which has plaster on one side and exposed brick on the other. I need to make the plaster side flush with the brick along the edge where it's now missing about 1/2". I can do it by using a 1x4 along the plaster edge as a mold and filling the gap with drywall mud but I'm afraid the mud will stick to the board and crumble when I take the mold off.

If I put aluminum flashing over the board before filling the gap with mud, will that prevent sticking?

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The drywall mud is just as likely to stick to the aluminum flashing as it is to the board. Just observe how well dried mud sticks to your drywall mud taping knives. It may be possible to put a light coat of mineral oil or silicon lubricant spray on the metal beforehand to minimize the mud adhesion. But you would definitely want to experiment with some small samples before committing to the whole corner.

I believe that you would be way better off trimming out that edge with an appropriate style of drywall edging and then applying the drywall mud up to and over the fin of the edging.

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(Image borrowed from ClarkDietrich)

The reason for this is due to the fact that if you try to build up a large thickness of drywall mud it will always be subject to damage and crumbling out when it gets hit from the corner.

Stability of the L-Stop Bead edge will likely require that you install a backer strip of wood along the edge of the corner to fill in the space.

  • I need to put some mud over the presently rounded corner in order for a corner bead to sit on tight. to accomplish that, I was going to set a board along one of the two edge's sides and secure it with clamps against an adjacent window, then fill the gap with mud. – amphibient Jun 24 '16 at 16:28
  • I understand. Maybe a quick coat of poly on the board would eliminate the mud adhesion. – Michael Karas Jun 24 '16 at 18:54
  • poly as in polyurethane ? – amphibient Jun 24 '16 at 19:36
  • Yes, that is what I was thinking. Then wax it nice. Test out first of course. – Michael Karas Jun 24 '16 at 19:38
  • what kind of wax should i use ? – amphibient Jun 24 '16 at 19:54

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