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How to patch cracks in plaster.....the pros methods........

Posted by Brian Ewing on September 25th, 1998 08:20 AM
In reply to ceiling with cracks by Ceiling on September 24th, 1998 11:58 PM [Go to top of thread]

23 of 26 people found this post helpful

First off let me say this:

DO NOT! Install tape or fiber-glass mesh over the cracks in your home!

This is a complete waiste of time and expense! No little bit of tape or fiber-glass is gonna stop the movement of your walls and if your walls do move the cracks are coming back!

Not only do you have the cracks back but you have to go through the extra time and money of having to deal with the tape and/or mesh.

Really.......It's just a waiste of time....

Now to fix your cracks.

I'll post here a post I created on my BB on repairing plaster cracks. It may be a bit long winded, but it is truly the correct method of restoring your plaster cracks.


For small, old age, expansion and contraction cracks obviously nothing like major movement of the walls, start at step 2a. For cracks that are deeper and obviously movement cracks follow step 1a (after you have determined that the movement has been mitigated).

For Major movement cracks:

1a: Using a screw driver, or other strong pointed tool, carve a slot on both sides of the crack, its full length. This slot should be about two or three inches from the crack on both sides. Its purpose is to isolate the crack from the surrounding wall.

1b: Remove all the plaster between the two slots. You need to go down to the lath. If you have old wooden lath this should be easy. For metal lath you most definitely will want to use a masons hammer. Just be sure you don't rip the original lath. Don't hit too hard, you can still spall the base coats of the surrounding wall from its lath.

1c: Brush out the area that you removed and get out any remaining dust, pebbles and what not. Also, for original metal lath, remove most of the plaster remaining in the little holes.

1d: If your lath is of the wooden variety. Install, using screws, a strip of "diamond mesh" metal lath going the full length of the opening. If you have to use more than one piece of lath be sure to over lap the pieces by at least two inches. Make sure that the lath is tight by pulling one end of the lath as you screw it to the wall. If your original lath is metal, you will need buy a spool of 19 gauge galvanized wire. Cut the wire into lengths of about five or six inches. Bend the wire about one or two inches from one end. Take the bent end and insert it through the new wire and through the original lath. Pull the short end back out the lath a few holes away and twist the wire with a pair of pliers until it is good and tight.

1e: Paint a Poly-vinyl-Acetate bonder over the edges of the opening. P.V.A. works great at keeping the old wall from sucking the water out of the wet patching material and at the same time creating a good bond.

1f: Buy a bag of "perlited - Structo Lite" (U.S.G.), some people would say that you should use "Red Top" plaster that mixes with sand, but I am not writing a book here. Mix the base coat as per the directions on the bag and trowel it into the opening about 1/4 to half way to the face of the surrounding wall. Cross rake this coat to roughen it up a bit. This is your scratch coat.

1g: The following day, mix another batch of plaster and fill the opening to about 1/16th inch or so from the surrounding wall face. The final 1/16th inch is to provide room for the finish coat. Leave this coat alittle rough. This is your brown coat.

1h: Give the brown coat time to cure out. About eight to ten days is good enough.

1i: Buy a bag of "Red Top - Slow Set" Finish Plaster (U.S.G.) and mix it as per the directions on the bag. Trowel on the finish plaster flush with the surrounding wall. Take a brush and wet the wall occasionally by dashing it with water from the brush and troweling until the wall is smooth. Be sure not to hit the wall with the brush as you do this.

1j: Being a "newbie" you may need to sand the patch after it dries.

You are now a plasterer!

For minor movement cracks:

2a: First open the crack with a tool such as an old screw driver. You need to make the crack about 1/4 inch wide.

2b: Brush out the opening to remove any deleterious dust particles and such.

2c: Apply P.V.A. to the opening to give the patching material a good bond.

2d: Buy a bag of "DuraBond90" (again, U.S.G.) and infill the crack flush with the surrounding wall. If the patching material shrinks and creates a pit where the patch is, just apply more material.
NOTE: The pros will probably use either lime/gaguing mix or RedTop mill-mixed plaster for the finish coat. However, since you are inexperienced you should use Durabond90 or EasySand90. It's not quite as good, but it will work. Just DO NOT use drywall mud!

2e: After the patch is dry, sand it smooth and flush with the surrounding wall.

I know that the first step seems hard. But, this is the proper way to patch a wall and
if its worth doing, its worth doing well and correct....

I hope I have helped some here........

Brian Ewing

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