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"Painting raw edge of plaster board"

August 3, 2018

Q. Hi. My bathroom renovation is almost complete. The frame and sill of a window (not in the shower) were not tiled with the tiles on the wall as they should have been. Instead, there is a layer of plaster and paint covering the cut raw edges of sheetrock and edge of tile. People who have seen this state that I should line the frame and sill with marble or Corian or similar material to protect this from water vapor damage.
Could a coat of water based polyurethane give sufficient water resistance or, better yet, waterproofing? Can it be placed over regular household latex paint? Some other materials (varnish, shellac, lacquer) I understand will yellow over time. I've looked into waterproofing paints and acrylic sheets but technical assistants for these products say they will not serve my intended purpose.
Thank you

Susan Jormark
- New York, New York

August 2018

A. Hi Susan. A pic would help, but I think I understand what you have. It's not as much a question of water vapor as of water from the window, mechanical damage, etc. Listen to the 'people who have seen this' -- the raw cut edge of plasterboard is not going to be acceptable no matter what you paint it with.
Whether you use a marble or Corian sill, or a wood sill, you need molding of some sort all around the window opening, no raw edges of plasterboard.


pic of Ted Mooney
Ted Mooney, P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey

August 9, 2018

Hi Susan,
The most bang for your buck both in terms of materials and labor (LOVE LOVE LOVE marble, but oof have fun cutting it!) is going to be trimming-out with white PVC board. More and more hardware stores and lumberyards carry it now. It's commonly used on patio railings and porch trim. I have a window IN my shower (downstairs bathroom used to be a Butler's pantry) that was rotted out and my dad I removed all the damaged wood, painted (or should I say messily smeared lol) the hemlock house framing underneath with 2-part polyester boatyard resin to stabilize the surface against dry rot, and rebuilt the decorative trim-boards and sill with PVC. It is very easy to work with, stands up brilliantly to moisture, and can be painted with normal latex paint. Also, silicone tub caulk sticks to it very well. Apply CONSERVATIVELY, bare minimum you absolutely need, removing all excess, before paint.
You'll be SO much happier for SO much longer if you do the job using traditional trim protocols and long-lasting materials!
Good luck!

Rachel Mackintosh
Plating Solutions Control Specialist /
Industrial Waste Water Treatment - Brattleboro, Vermont


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