There are a lot of home improvement and repair jobs that are better left to professionals. Fixing a crack in the drywall is not one of them. A professional can charge hundreds of dollars, but it's an easy and inexpensive DIY job.
Cracks routinely appear in drywall, particularly over doors, when a house settles. To fix it requires a few supplies and steps. Always start wall repair and repainting jobs by protecting your woodwork and molding with painter's tape, and throw a drop cloth on the floor.
Open up that crack with a flathead screwdriver. Just scrape away everything until you reveal the drywall joint. Clean out the crack well and get rid of all the dust with a tack cloth or old toothbrush. The less dust, the better your repairs will adhere.
The real trick to solve the problem is self-adhesive fiberglass mesh tape. This tape will help reinforce that joint to prevent any more cracks in the future. Cut a strip of tape and cover up that joint you just cleaned off.
Now you can apply the joint compound. Spread it out in very thin layers, feathering the compound out and away from the cracks. Three layers should do it. Let each layer dry well before applying the next. Scrape the surface smooth with a drywall knife and dust it off between each layer.
After the final layer of joint compound is dry and scraped, sand it smooth with a fine grit sandpaper. After you've cleaned off all the dust one last time you're ready for priming and painting.
Don't skip the primer-- it will help the new coat of paint blend in better with the old coat.
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