take a stick, like a yardstick and break off a piece and tie a string onto the center of it... put the stick into the hole and pull on the string so the stick backed up the the hole in the wall and then apply some sheet rock patching compound into the hole area while pulling on the string.. this will keep the mud from falling into the hole and stay where its suppose to the the drilled area...... when dry cut off string and sand the area flush with the rest of the wall.... that is what i did when drilling holes and fishing for burglary alarm wires and it worked fine.... might take two applications of the mud as the mud tends to shrink when drying....
Material falling thru hole is not a problem because insulation packs tightly inside backing up hole. My concern is cracking around 1" hole unless taped. I would like to "dish" out hole and an inch or so out, tape, then finish level with surrounding surface.
Patch with tape and lite drywall compound (cut at least 10% with water). Feather out the mud.
Another alternative ... make the holes a little bigger and use a California patch. You cut the perimeter of the drywall an inch or so larger than the hole you're patching. Score the drywall to size and snap off the edges LEAVING THE PAPER BACKING so that your patch fits the hole and has a surrounding 1" or so of paper. Fit it in, using the paper flap as "tape", mud and feather.
What I have done in the past is cut a piece of drywall a little larger than your opening, for you that would be a
1 1/2 to 2" circle. Remove enough plaster around the edge leaving the paper intact to form a plug. Now do the hole and string thing to keep tension on the plug, and mud it in place. If you do it enough you can cut a little edge in the wall to sit the paper portion of the plug level with the existing plaster, thereby making a better looking job. It takes time and patience, but around the time you're done you know exactly how to do it right.
I was able to fill 1" diameter holes with just spackle and nothing special. Try to apply the spackle or plaster onto the inside surface of the holes (very roughly making the hole smaller) rather than just mushing it down into the hole. It takes two or three applications per hole, allowing drying between applications, before it can be filled.
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