I've developed cracks along the seams in the drywall on my ceiling. The house is old, so it might be settling. What's the best way to fill them and how do I do it properly so they don't come back?
Reinforcing mesh with subsequent spackling - that's the right way to do it. Just filling in the crack won't help, especially if your house is actually settling: the spackling will just crack again. You need to open up the crack, lay down drywall tape (a special mesh tape), then apply a layer of spackling compound, let it dry, sand it, and apply a finish coat.
The key thing is don't skimp on prep work. If the cracks are along the seams, chances are the drywall wasn't installed properly in the first place or the seams weren't sealed right. Before you put down the mesh, check if there are any voids under the drywall - tap on it, and if it sounds hollow, that's a problem. If your ceiling really keeps moving from settling, that's actually a major issue, and using caulk between the sheets might help as a precaution, but there's no guarantee.
Joint compound for drywall with reinforcing tape (mesh tape) is the most reliable option, but if the cracks are along the seams - specifically along the seams - then the problem is most likely that during the initial ceiling finishing, either the joint wasn't taped properly or the tape was applied crooked. Fixing this completely is trickier than it sounds - you need to remove the old layer, apply the mesh tape straight, and put down several coats of joint compound with sanding between each one, otherwise the cracks will come back whenever there's any settling.
If the house is really old and is settling, then even perfect initial finishing won't guarantee the cracks won't reappear over time - in that case it makes sense either to accept that you'll need to do periodic seam repairs, or to consider replacing the drywall ceiling with a stretch ceiling if your budget allows for it.
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