Here's my dilemma. Three years ago I hired someone to sheetrock the upstairs bedrooms in my Cape Cod style home. It might be hard to see in the pics I've provided but the seams where the sloped ceiling meets the flat ceiling and where it meets the walls are quite wavy. I was a little annoyed when the job was finished but I didn't think I could have done a better job. My family had also used this person many times before and the work was always top notch, so I didn't make a huge fuss.

Right now I'm in the process of turning one of the rooms into a nursery and the wavy seams are starting to annoy me more and more. I have learned that by using shims I could have made these seems nice and straight, but there is no way my wife will let me remove the sheetrock with the baby due in two months!

Is there a way to correct these seams without removing the sheetrock?

The only way I know is to build it up with more mud and sand it smooth...

Ask your wife to turn it into something cute, with paint or paper borders or glow in the dark stars or mobiles or peel off Disney characters. Don't worry about wavy lines. It looks like it would have been 5 times the cost to get it all flat in the beginning. I think it has character. Wavy lines - how about water scenes? Captain Hook, Tinkerbell, the Little Mermaid, Finding Nemo.

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I like the idea of using the line. I was thinking of something along these lines (no pun intended) but I couldn't picture how to set it up so that I can get a perfectly straight line. I will try drawing the line like you showed. Then I will temporarily nail up a scrap 1x to rest the edge of the putty knife against when I'm smoothing out the compound. If I use a 10" knife I should be able to completely straighten up those areas. I'll try this on one area first and then report back to you guys.

Thanks for pointing me in the right direction.

Snap a chalk line. Drive a small nail at one end. Tie chalk line to nail, (or have some one hold it) pull tight and snap. Using 6" dry wall taping knife, fill in corner. Put knife on line and feather out about 6 inches. Fill in about 12 to 15 liner inches, then place corners of knife on line and wall (or ceiling) and smooth. The corner of the knife sliding along the wall will be your guide. Smooth from end of mud backwards. Hold knife at about a 45 degree angle. You will be cutting off the mud evenly, not pressing it smooth. Holding the knife too flat causes air bubbles. Smooth back into what you just did. You can't put the knife down on the mud without leaving a mark but you can lift it without leaving a mark. Don't over work. The mud begins to dry as soon as you put it on the wall.
Plan on at least three coats. First coat will be thick. Don't try to get it too smooth. Let it dry at least two days. Second coat will not be as thick and will be easier to put on smoothly and should dry in one day. Second coat should extend out beyond than the first. If necessary sand between coats.
If necessary, you can put on more than three coats. It may be necessary to add a little water to the mud when doing finish coats because you will be putting it on and scraping it off. Not much, about two tablespoons to a pan of mud, you don't want the mud so thin it falls off the knife.
When you are satisfied, sand and paint so you can see your flaws then put on a final coat to fix flaws.