There seems to be a different method for each post
My contractor neighbor says that he fills the cracks with the vinyl based spackle but I've never heard this. someone else said use the slower drying plaster of paris (there's fast and slow)
Some others say if it's down to the lathe then you have to used the silica based scratch coat. Do I use the fiberglass tape over the wooden lathe if I happen to get down that far or just use the silica based scratch coat?
Thanks
Just filling a crack with vinyl spackle will virtualy guarantee the crack returns. Proper fix depends on how wide the crack is.
If it is just a hairline crack, not wide enough to really be able to force any patching material into, then I like to tape and mud it just like it was a drywall seam.
If it is wider than hairline, then you should dig out the plaster ~1/4 inch wide for the length of the crack. Use a utility knife to do this, and make an "inverted V" shape, in other words it is dug out wider at the bottom of the crack than at the wall surface. This is so that when you fill the crack, the new plaster can't fall out. Fill with patching plaster found at any hardware store. Sometimes I still like to tape and mud over this patch also.
If we are talking about larger areas, sometimes you can get way with just continuing to use a single layer of patching plaster brought flush with the wall surface. Better is to do it in two coats, first coat scratching into the lath to key it well, second coat to bring flush with the wall surface. I've sucessfully patched 1 foot square areas this way using just plain old patching plaster.
If your area to be patched is even bigger, then you ought to consider getting real base coat plaster and white coat plaster.
Often I will still tape and mud the junctions between old and new plaster to help prevent future cracks.
Still another thing to look for is loose plaster. You may have just a hairline crack, but the plaster on one or both sides of it may be loose. Check by pressing gently along the crack to see if any plaster is loose. Often times the plaster keys will have broken off inside the wall leaving you with a nice solid piece of plaster that isn't really attached to anything. In this case, use plaster washers to screw down the loose chunks of plaster before you do annything else. Try to put the screws into a stud wherever possible, but you can also just use short 1" drywall screws to screw the washer into the lath alone. This helps, but is not as good as hitting a stud with a 1 5/8" or 2" screw.
If you have sections of loose plaster that can't readily be screwed down with plaster washers, then I just knock out those sections and go ahead and patch the newly created hole. In the long run it's easier than trying to fool around with reattaching a 6" square chunk of loose plaster.
Finally, after patching with whatever plaster you choose, I will sometimes go over the whole thing with drying-type drywall mud so that I can sand it and make a nice smooth surface. You can feather out drywall mud, but you can't feather plaster. Many times if you have a crack, the plaster has started to pull away from the lath a little bit creating a high spot. Even if you go ahead and cut out a notch and patch it, you might still be left with a high spot that looks bad. feather over it with drywall mud can help ease the transition.
Ken
I just tried vinyl spackle for the first time recently, and it was like trying to squish a tennis ball down a drain :o) If the cracks are fine and the structure fairly stable, a paintable caulk is best - flexible so it does not crack soon again, invisible when painted. If it opens up again, then you need to look for what is moving.
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