I have been repairing wallboard cracks myself today. I moved into a house (my first) a year ago. I immediately painted one of the spare bedrooms, which had a few cracks in the drywall above doors and at window corners. I don't remember for sure what I used but I think it was spackling putty. All I did was spread it on and try to even it out. After about six months about half the cracks came back.
So now I am trying doing it the right way, in both that room and another spare bedroom that is even worse crack-wise. There are some cracks too along the corners both with the cieling/wall junction and up and down between two walls. Last year I marked my walls with some pieces of tape at corners to guage wall movement. I was afraid my house was settling in one direction, but turned out it moves one way in the summer (hot no rain at all for six or seven months, then rain like a banshee in the winter)and back the other way in the winter. I don't think that the previous owner ever cleaned the gutters (four inches of dirt like material in them all the way), and the extensions were missing, water was dumping right onto the house edges. I rectified those problems in the hope of maybe reducing this seasonal movement.
Anyway, now I'm digging out the cracks with a utility knife, putting down a thin layer of joint compound and putting a piece of the self sticking "fibra" or soemthing like that wallboard mesh tape. Letting it dry and then putting on more thin layers and going further out until with sanding I can get it to blend in smoothly with the wall.
In the corners at the wall junctions and wall to ceiling junctions, I'm trying something different. I have read of some people doing it, and others say no, but I'm trying it. I dug out the cracks with the utility knife. I got some elastomeric paintable caulking and ran it into the corners, and smoothed it out with my finger. Almost seemed too easy.
It's going to look good, but I'll have to check back in a year to say if the crack repairs hold this time!
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P.S thanks to whoever it was who mentioned the name for the "curved wallboard knife" as I had read about it somewhere and it sounded like it would make things much easier. I went to hardware store and no one knew what I was talking about, but I couldn't remember the name of the tool or where I had seen it. Now I know the name.