RTV, silicone caulk, "Shoe Goo", possibly bonding using a piece of fabric and a solvent to "weld" the crack, epoxy, Gorilla Glue, or any combination of the above. Without knowing what the barrel is made of (ABS, PVC, etc), it's hard to make a definitive recommendation. The surface you are going to repair needs to be squeaky clean. Get some sandpaper and rough up the surface around the crack before you start, otherwise anything you put on it will have a hard time bonding to the plastic. Usually, the way to keep a crack from getting worse is to "stop drill" it. That is, drill a very small hole at the exact end of the crack (at each end). This removes the concentration of stress and reduces (but not eliminates) the chance that the crack will get bigger just by normal flexing. Of course, if your goal is to seal the crack, stop-drilling just means you have two holes you have to seal as well.

sfsuperstar: "My husband's really doesn't like the ridge, and the rest of the ceiling is so smooth he thinks trying to blend a 3/4" deep ridge will look obvious, so his tentative plan as of this evening is to remove the ridge of mesh to try to flatten it, then try to redo the crack repair smooth." That sounds reasonable. You may have to widen the crack just a bit more and clean it out, trying to cut under the edges as much as you can so the patching material has a place to grab in to and hang on. sfsuperstar: "It seems that the sagging part isn't an edge of the mesh, but is a center part of the mesh -- like an upside-down mini-mountain ridge. Should he try to cut out just the ridge part of the mesh with an exacto knife, or does he have to remove the whole mesh tape (even the part that's not sagging)?' I would guess it depends on how easily it cuts. I've not used it before, so I can't say. If it ravels and gets frayed and stringy, then it may be hard to cover over again with whatever you use. sfsuperstar: "And should he re-repair the crack / removal damage with flat drywall tape and joint compound? Or is there something plaster-specific that he should use? I'm not familiar with the patching plaster you mentioned." It may be OK to use the drywall tape, but I would wonder how much it would give or move under changes in temp and humidity in the ceiling without breaking loose at some point. I really don't know! We got the patching plaster at the hardware store; it's dry and comes in fairly small bags, and you just add it to a small amount of water in a container and let it absorb the water, stirring or 'folding' the mixture only very gently. If memory serves, drywall compound is still water soluble after it dries, while the patching plaster is a bit more impervious to water once it dries. Drywall compound also has a tendency to crack easily as it dries if it's applied to thickly, and the patching plaster doesn't do that as readily.

More specific answer -- Old Plaster has a tendency to show cracks. You can just paint over it, which many old house owners do or do a skim coat of new plaster, hard to work with, or joint compount, preferred over the small cracks to make it smooth again. A skim coat is just a very light layer of compound that you smooth over teh existing layer and then sand with a high grit sandpaper to get a smooth surface. My prefered method when I have time is to do light weight joint compound where teh crack is,you are really just putting a very thin coat on with the knife, let it dry and then sand with an orbital sander attached to a shop vac. Looks great and no mess. As i rarely have time, I more often just paint over the cracks and don't bnotice them except when I am being picky. Is the areas that is stinky over the crawlspace, if so thats another advantage to completely sealing the crawlspace floor -- non-stinkiness. The keeping of moist vapor out should also help the smell. Caulk or expanding foam should also help, but I would think that stopping the source of the problem would be much more effective and they might not do much by themselves.

> Old home built just after WW2 I've never lived in an old house with plaster walls and ceilings that didn't have cracks. They might not have been immediately visible when I moved in, but they were there all right, hidden under a layer of spackle or plaster patch. My guess is that your swirl texture is a recent addition, and that your crack was there and was patched before the texture was applied. Plaster cracks are a fact of life in an old house. You patch them when you paint. Sometimes they return. Sometimes they'd don't. It depends on whether that part of the house is still feeling restless, or has settled into its bed for a nice long snooze. :) You can demolish all the old plaster and replace it with drywall. Then you won't have plaster cracks any more. Unfortunately, you also won't have plaster any more.