It was applied in too warm a condition so it dried to quickly or applied in too cold condition and the frost effected the curing and subsequently caused the cement to fail and hence has no strength. The topping coat is a stronger mix than the scratch coat which means the base coat willl be able to flex.
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Many things can cause plaster around a window to crack or crumble, but repairing it doesn’t require resurfacing the whole wall. If the plaster only has a few thin or hairline cracks, you often can disguise them with flexible acrylic painter’s caulk, then paint over the repair. If the damage is more severe, you need to remove the trim molding and replace the damaged area with new plaster. Drywall joint compound usually is not the best choice for plaster repairs, and neither is unmodified plaster of Paris. Choose plaster patching compound, which is available in most home improvement centers.
1
Pry off the trim molding around the window with the flat edge of a pry bar.
2
Pull the nails from the molding with a nail puller or pliers. If necessary, tap the tip of the nails with a hammer to push the nail heads out from the surface of the molding enough to remove them with a puller or pliers.
3
Scrape out damaged plaster around the window with the flat edge of the pry bar or a metal 5-in-1 scraping tool.
4
Sand the plaster around the area where you removed the trim with coarse-grit sandpaper. This roughens the surface and helps the repair compound hold. Sand approximately 3 inches over onto the plaster.
5
Vacuum debris from the spaces around the window where you removed the molding and from the plaster you sanded.
6
Mix plaster patching and repair compound with water in a plastic container such as a bucket or large bowl that is not used for food preparation. You can mix a small batch with a wooden paint stir stick; large batches more easily are mixed with a paint mixing paddle fastened to a power drill. Use the dry mix-to-water ratio recommended on the bag, and only mix as much as you need to apply one coat. Mixed plaster compound does not stay wet when stored. The thickness of the first layer is not critical. You need enough to force some through the spaces between the lath strips while leaving approximately 1/2 inch of material on the face of the lath.
7
Scoop the compound onto a plaster finishing trowel or drywall finishing knife. Spread a 3/8-inch-thick layer of compound onto the lath, gently pressing to force some of the material between the lath strips; this creates lumps called keys on the back side of the lath, which hold the plaster in place after it dries.
8
Wait 10 to 20 minutes until the plaster feels solid, cold and damp, but not hard, then drag a plaster comb across the surface of the plaster to create ridges. If you don't have a plaster comb, you can create the ridges by dragging a thin finishing nail across the plaster in numerous rows approximately 1/16-inch apart.
9
Let the first application of plaster patching compound dry for as long as the manufacturer recommends. In approximately two hours, the product will harden enough to apply more.
10
Mix another batch of compound when the first layer is hardened to the touch. Spread a 3/8-inch-thick layer over the first layer, comb or score it very lightly with the comb or nail, and let the repair dry.
11
Mix a final batch of plaster after the last coat you applied feels hard. In some cases, this requires overnight curing, but you may apply the finish coat the same day if the plaster patch compound manufacturer directs you to do so.
12
Spread a thin layer of plaster onto the repaired area and over onto the sanded portion of the surrounding plaster, making a seamless repair. This layer is a skim coat, and should be approximately 1/8-inch thick. If 1/8 inch is not enough plaster to match the thickness and blend with the surrounding plaster wall, apply a thicker layer.
13
Mist the fresh plaster on the wall with water from a pump spray bottle, and feather or blend the material with the flat side of the blade to create a smooth finish. Unlike drywall, which is sanded to smooth out imperfections, plaster is smoothed and finished with the metal blade.
14
Select finishing nails slightly larger in diameter than the original finishing nails used to install the molding.
15
Hold one section of trim molding against the edge of the window frame where you removed it. Drive finishing nails through the original nail holes in the molding and into the wall. Install each section of molding the same way.
Things You Will Need
Pry bar
Nail puller or pliers
Hammer
5-in-1 scraping tool
Coarse-grit sandpaper
Utility vacuum cleaner with hose
Plaster patching compound, dry mix
Plastic bucket or bowl
Wooden paint stir stick
Paint mixing paddle
Power drill
Plaster finishing trowel or drywall finishing knife
Plaster comb or nail
Pump spray bottle
Tip
The exposed wood lath in the wall is your guide for applying the patching plaster. If you keep the plaster on the lath, you will not come too close to the window jamb.
About the Author
Carole Oldroyd, a writer based in East Tennessee, has authored numerous DIY home improvement, Human Resources, HR and Law articles. In addition to holding a degree in paralegal studies, she has more than 10 years of experience renovating newer homes and restoring historic property.
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Carroll, Lee. 'How to Repair the Plaster Around a Window.' Home Guides | SF Gate, http://homeguides.sfgate.com/repair-plaster-around-window-38254.html. Accessed 01 November 2019.
Carroll, Lee. (n.d.). How to Repair the Plaster Around a Window. Home Guides | SF Gate. Retrieved from http://homeguides.sfgate.com/repair-plaster-around-window-38254.html
Carroll, Lee. 'How to Repair the Plaster Around a Window' accessed November 01, 2019. http://homeguides.sfgate.com/repair-plaster-around-window-38254.html
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