I'm finishing drywall and am having a problem with the inside corners... I used the 90 minute Sheetrock Lightweight Setting-Type Joing Compound powder to embed the tape everywhere... all that worked great...

However, I'm finishing with Sheetrock Lightweight All Purpose Joint Compound (blue lid). I applied a thin coat on each side of the corner, about 1/4" from the corner... then, when dry, use a corner tool to fill in the corner.

Everything looks great when I'm done applying but, once it dries, the corners have tiny hairline cracks down the corners. It looks like the compound just cracked upon drying, but I can't figure out why... the thickness of the corner coat is maybe 1/16" or less... definitely no more than 1/8" anywhere.

Anyone else have this problem? Any ideas on how to fix it? I was thinking MAYBE I thinned the compound too much? I didn't add that much water, but that's all I can think of. Or maybe I should run a wet sponge down the corner before I apply the corner coat to keep it from drying too fast??

Any help would be greatly appreciated.

Hairline cracks usually means it was too thick. You can do one more very thin coat or take a damp sponge and wipe the corner. I use a damp sponge rather than sand paper to feather my edges, no dust or scratches

Thanks for your quick response. I assume the wet sponge will serve to "re-melt" the corner compound to fill the cracks? I'll try both suggestions - thanks again so much.

It probably is shrinkage if you are thinning the joint compound very much. The corner tool is probably pushing all of the mud out of the corner and you are not filling the crack. On the corners you have done, run a small bead of caulking in the crack and wipe away the excess with your finger. Throw away the corner tool and do one side of the corner today and the other tomorrow using your joint knife. Using a wet sponge is like sanding but the dissolved joint compound does not re adhere. It goes in the bucket you rinse the sponge in.

Yes, it remelts and cause the compound to flash or spread amd smear out smooth