On Feb 2, 11:09 pm, "MiamiCuse" wrote:
I am remodeling this 35 year old house and throughout the house the walls
are made of a double layer of gypsum board as the layer underneath and a
layer of plaster / cement material on top. Total thickness is about 3/4".

http://i173.photobucket.com/albums/w...e/P1010626.jpg

This presents a problem in matching new wall with existing. If I use the
standard 3/8" or 1/2" wall board it will not match. I could use them in
areas where I torn down the entire wall from end to end, but in other areas
I have to match the existing thickness. I guess I have three options:

(1) Use one layer of 3/4" sheetrock and try to match it, or
(2) Use two layers of 3/8" sheetrock, or
(3) Furr out wall a bit to use one layer 3/8" wall board

Not sure which one is the most convenient and economical.

Also, electricians, plumbers, window installers have worked on the house and
there are numerous places I need to make a patch but the patching is
impossible since the cuts are done in an irregular fashion. See pic:

http://i173.photobucket.com/albums/w...e/P1010625.jpg

I try to cut them on a straight line and it's hopeless. Can't use a
recipricating saw because of the concrete layer underneath. Tried a angle
grinder and there were so much dust I could not stand it. Any advise?

Yep. Don't let your tradesmen cut nasty holes for you to patch. You
should cut the openings or have the tradesmen cut straight lines.

A reciprocating saw will work just fine, you'll just go through a lot
of blades. Most reciprocating saws accept the blade in a reversed
position (teeth are facing up instead of down when the saw is held
horizontally). Reversing the blade allows you to have a much lower
angle of attack which is necessary since you don't have blade
clearance behind the drywall. Tape the vacuum hose to the end of the
saw so it will pick up most of the finer dust as you cut.

As an alternative, it's easier to cut complicated outlines in the new
drywall than trying to cut into the existing wall. You'll have to
clean up the edges of the cutouts, but you can use scribe cutting
techniques to cut any sort of outline to follow the contours of the
cutout. You mark the scribe line on the new drywall patch with a
compass and cut to the line with a drywall keyhole saw. It goes
fairly quickly and will cut way down on the amount of dust you'd be
blowing around cutting the drywall with a power tool.

I attach scrap plywood blocking behind the drywall so it's fastened on
both sides of the cutout every foot or so (the blocking ends up
looking like a ladder running up inside a long cutout). Then you can
attach your drywall, or cardboard/plywood shims, to the blocking. I
prefer to have the cutout repair drywall sit just a bit lower than the
original wall surface as it makes taping easier. You'll end up
bridging over the entire cutout and it's better to have one smooth,
gradual bump, than to have bumps on either side of the cutout from the
tape.

I would not use a single layer of 3/8" drywall. 5/8" should work
nicely. If the existing wall surface is flat and true, you could shim
that out a bit with some cardboard to you won't need as much joint
compound. Standard fiberglass mesh tape would be a poor choice for
your repair. It's noticeably thicker than paper joint tape and the
loose ends end up requiring more compound to cover them (and you can't
sand them), so the bump ends up being bigger. Either use paper tape,
or something like Tuffglass tape (TKO Coatings) - that's a flat woven
tape that is much thinner than standard fiberglass tape.

R