Patching and Plastering - Patching drywall

I had to cut a 3 in x 3 in hole in the drywall to run wiring throught a fire wall. Can i use the painted piece of drywall that I cut out or should I satrt from scratch. THere is a texture on the existing wall sso it would be better if I could use the existing piece. Also, when i attach the patch piece can i use drywall mud or do I have to use some sort of fastener? Any help or suggestions would be appreciated.

Your best bet would be to use the piece you cut out. If there is no stud where you cut the hole, you will need to add support. Put a couple of pieces of 1x2" on each side of the hole so that the length of the pieces is a little more than the size of your hole and then attach them with screws through the existing drywall (you'll have to hold them in place with one hand and have your drill or screwdriver in the other) so that half the width of the board is behind the drywall and half is hanging over the hole. Then attach your cut out piece of drywall to the 1x2"s with screws. You'll have to do some mudding, then prime and paint.

...or sometimes I'll use a good construction adhesive to attach the backer boards to the inside of the wall.. let that dry solid, which it will, then attach your cut-out piece either the same way. I really like the Titebond Construction Adhesive..sets up pretty fast and when dry, pretty solid.
Less screw holes to fill...since its a textured wall...

first use the piece that you cut out, use a bit of sand paper and sand round the piece you cut out to slightly reduce the diametre. then making sure you have brushed the outside of the hole with pva apply a small amount of drywall adhesive around the hole, you can get it from b and q. then apply the piece you cut out back into the hole level with the wall. use a wet brush to take away any exces lumps of drywall adhesive that may be on the wall and leave to dry solid, then simply paint the wall.

As soon as I can figure out what PVA is I'm gonna' head on down to the local b and q and get me some. Anyone know where I can find a b and q?

I'm assuming PVA and b and q are stuff known in the trades, but you guys have to take it easy on the rest of us DIYers. Sometimes it's better just to spell it out.

Seriously, how do you get a smooth paintable surface w/o sanding? I can do a taped patch and a hot patch, but this is the first I've heard of a glue patch. Do you use a backer behind the plug?

Wayne,

Describe your texture. This is the most important part. Fixing the drywall can be done different ways...but you don't want to see no ring around where the patch is. And just fitting back in your piece...you are going to see a ring. You are at the least, going to have to take a cotton swab or toothpick or something (depending on the texture), and daub some mud on that ring to simulate the texture before painting.

Or, if you have "orange peel" texture...they sell this in spray cans at the big home centers, and you can then piece in even new sheetrock and then respray it...feather edging it outward over the existing textured wall to hide the repair. I have done numerous of these repairs and you can't see them. [You wouldn't believe all the repairs, of various sorts, I have done in college (some from the 1800s) and newer rental houses for about the last 20 years]