Making Plaster Crown Moulding - HELP!


Old 11-01-02, 06:15 AM
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Angry Making Plaster Crown Moulding - HELP!

My neighbor and I are trying to replicate his old crown mouldings made of plaster.

Here's what we've done so far;
- made an exact template of the moulding
- built an 8 foot trough,
- fill it with plaster (structo-lite),
- drag the template across until plaster is smooth, and
- let the piece dry.

About 7-8 hours into the drying process the plaster will either start to crack, or it will rise up in the trough and begin to twist.

The cracks aren't too bad, we can fill them; but the twisted pieces have to go into the trash.

We've tried mixing less water, raising and lowering room temperature, but are getting fustrated. His basement is clean and dry, about 68-70 degrees.

He saw this done on TV, This Old House? and HE thought it would be a relaxing project.

It's really not all that bad; but there's 40-50 feet to still make.

What are we doing wrong to cause these cracks and twists?
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Old 11-05-02, 08:31 AM
Join Date: Nov 2001
Location: Taylors, SC
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I was surprised at how much and how little information was available on the WWW on this subject.

From a furniture maker's view, I would suspect that moisture it the essence of the problem. You have tried less water, you might try more.

Some of the moldings that I read about were cast in place; that is, they were made on the wall with a build up of plaster that was molded in place. I suppose using a template such as you describe.

Other sites discuss laying up the plaster in layers into a mold. It seemed that timing was critical in this process.

Some of the sites led me to believe that plaster for such a purpose should set in 30 minutes or so.

The manufacturers' sites talk specifically about how much water goes with how much plaster. And there is a certain amount of heat to contend with when the plaster sets. When the matter of water movement through the material is discussed, there is some point made about sufficient ambient heat to move the water from the plaster.

All in all, this seems to be an interesting type of work, requiring a fair amount of knowledge and skill.
Old 11-05-02, 09:21 AM
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Smile Found out .....

...... that the secret is to use plaster of paris.
Old 11-14-02, 07:21 PM
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the real secret is to use molding plaster. you have to use the exact plaster and water by lbs, everytime or it will crack.

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