In one of the bedrooms in my apartment under the window, part of the bottom of the ledge has collapsed. Not sure what caused it. Not sure what it is made of either, probably concrete or something similar. At its widest the hole is about 3 cm, which is a little too wide to fill with just spackle. I was wondering if I could patch the hole with duct tape and then add the spackle over it so it has something to adhere to?
Duct tape + spackle to fix hole under window ledge
duct-tapehole-repairspackling-paste
Related Solutions
I don't think you'll be happy with spackle for anything bigger across than a nail hole. It's not very strong, and also not very sticky. So what tends to happen in your situation is that the spackle will fall through the hole into the wall; if not when you're applying it then it will be likely to do it when you try to sand it smooth before you paint.
I think the solution that's going to get you the best results for holes that size would be to cut a 2" square of fiberglass drywall joint tape and stick it over the hole, and then use joint compound to fill the hole and cover the tape, feathering it out to a 6-8" diameter circle. You can do this reasonably well with a cheap 4" putty knife. Then sand it smooth, being careful not to sand all the way through to the tape.
Get yourself a drywall sander
Sand down the high spots using rough grit (100 grit) sand paper (they sell it to fit the sander - you'll find the sander in the drywall area and the sandpaper in paints) then smooth it with 200 grit.
If you have low points, fill them in after sanding.
Next time use less mud (little applications, not big ones) and then use a really really big sponge that's been slightly dampened to even it out.
Best Answer
Nope. Use proper drywall tape of either the paper or fiberglass varieties. Both are cheap and won't leave you with a gooey, hollow, fragile mess. I also recommend proper joint compound. Spackle is very light and soft and not suitable for filling large depressions. If you're dealing with concrete or regular moisture, consider a setting-type compound for toughness. (It comes as a dry powder and requires water.)