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I just read a blog stating that pre-mixed joint compound should not be put over pre-existing plaster walls. Why? What is the best product to use?

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Are you doing a skim coat, or patching holes/cracks? –  Tester101 Apr 13 '12 at 18:49
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The answer really depends on what you are trying to do. A full skim coat, patching, or covering lead paint. In general, many GP premixed compounds specifically say not to be used as a skim coat.

For the sake of argument, let's assume you are doing a full skim coat. Start with a full bodied joint compound, not a lightweight. Divide the pail in half into another container. Cut it a bit with water and Ivory dish soap, mix extremely well until it is the consistency of pudding. Firm enough to stay on your hawk, but loose enough to glide on the wall. This mix will elimamate most of the air bubbles and give you a very smooth coat without a lot of sanding. Apply with a 12 to 16 trowel or knife. Make your first coat about 1/16" thick. Sand it with a very fine sanding sponge or screen. Give it a second coat to fill the void spots and sand it again. You can wet float it for a glass like finish.

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Use Durabond; remove any loose plaster, then prefill holes and let the Durabond set. Once set then mud and tape the prefilled hole spots and let it set. As far as spider cracks and straight line cracks take the tip of a screwdriver, line it up with the crack and slightly apply pressure and drag a concave little trench so the Durabond has a base to adhere to. If you do not do this the cracks can re-appear! Note: tape larger cracks. smaller cracks can usually be strung over. Mud over what you taped and let set. Note: float tape and feather in outside edges, let set well, then scrape any high spots flat, then recoat. Depending on your skill at this, you may need to skim (lightly coat and sand) the entire wall.

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