If you bought a pre-owned home, particularly one that had a lot of DIY work done, you will usually notice areas where some drywall work was done and the taping and finishing sticks up as "bumps" or "humps". You can make these humps become far less obvious or even make them seem to disappear by using the procedure below.
Step 1 - Place coats on each side of the hump. Make the coats at least as high as the hump. Let these coats dry. See below.
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Step 2 - Scrape off the previous coat so it is level and free of any ridges sticking up (do not sand). Place a coat in the middle that bridges the two previous coats. Let this coat dry. See below.
Step 3 - Scrape the previous coat. Apply more coats to edges. You may very well end up with a final tapered coat that is more than 24" wide, but you should not see much of the humps! See illustration below.
Step 4 - Inspect with trouble light or lamp held at low angle. Add more thin coats as needed. Avoid any sanding until ALL coats are on and then only sand lightly. Instead of sanding, scrape down previous coats with the edge of your taping knife to knock off any ridges. Fixing these types of humps takes some patience!