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Old 06-04-2009, 09:12 PM   #1
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Prmier old plaster before patching?


Its been recommended to me that I prime my old paster with a shellac-based or alkyd primer prior to patching with setting compound. Is this extra step and expense necessary?

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Old 06-05-2009, 12:42 AM   #2
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Prmier old plaster before patching?


Since nobody else posted up, I guess I will. I'm far from an expert, but have learned my lessons the hard way. I use a deep sealer and a bonding agent (from now on, read my post minor catastrophy) anytime I do plaster repairs.
Plaster work is just too dirty and time consuming to skip any steps. In fact, in my experience, overkill is the best policy.
If in doubt I wash down the entire wall to remove as much wall paper paste, loose material and dust as possible and use plastic mesh or even metal mesh to reinforce problem areas.
IMO you have to figure it cracked for a reason. Framing does shift and shrink. Brick or block does expand and contract. Even dissimilar plasters expand and contract at different rates and/or absorb moisture in different quantities (humidity).
When patches are applied, the existing plaster can suck the moisture out of the patching material very quickly. This can cause cracks or mess with the bond. Though this seems to be less of an issue with improved materials in recent years.

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Old 06-05-2009, 10:30 AM   #3
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Prmier old plaster before patching?


I would not prime the old plaster first. It will inhibit the bond between the patch and the original plaster. You could dampen the surface of the old plaster to keep it fom drying out your patch too quickly if that's a concern.
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Old 06-05-2009, 01:28 PM   #4
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Prmier old plaster before patching?


Quote:
Originally Posted by Maintenance 6 View Post
I would not prime the old plaster first. It will inhibit the bond between the patch and the original plaster. You could dampen the surface of the old plaster to keep it fom drying out your patch too quickly if that's a concern.
I'd done hundreds of patches with few problems, using just the plaster and water. Though thicker patches did tend to crack, if they were not sprayed or dampened every couple of hours, so they cure slowly. This seems less of an issue recently, the materials seem to have been improved some.
A deep sealer, helps keep patches from drying to quickly, soaks in and stabilizes the existing plaster and may also help with the bond. I spray it on with an old widow cleaner spray bottle.
Using a brush on bonding agent, for me at least, is more of a why not thing than a why thing anymore. A half gallon is cheap and goes a long ways and can be used for other projects.

Last edited by Bigfoot; 06-05-2009 at 01:34 PM.
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Old 06-06-2009, 04:31 PM   #5
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Prmier old plaster before patching?


They have a product called, Plaster Weld, made especially for this task.
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