You need to remove it and start over. A coating of mud over standard drywall isn't going to stick long unless you also embedded mesh with it to give it strength. The next time someone slams a door or you try to drill through it, you'll have chunks falling off.

Renosarefun:

I'm going to assume you've taped between the old and new before feathering with the mud. Remember that working on a ladder, you've got a view of this repair that no one else will ever have. As long as you're not using glossy paint on your ceiling, a little undulation will hardly be noticeable. I wouldn't worry about it.

hollysprings;

Thanks for the reply, I'm not using standard pre mix drywall compound. I'm using a professional grade sheetrock 90 which is premium setting type compound which is not premixed, low shrinkage, crack resistance, much harder set then standard pre mix and feature exceptional bond strength (virtually unaffected by humidity). Most of my previous sentence was a direct quote.

I've used this compound on several other reno's and have never had any issues in years, even with large fills.

Trebruchet;

Thanks for the reply.

I've tapped with fiberglass mesh, but my main concern is how far back will you be able to see with a 1" gap along the top of the crown since this compound is hard to work with and my arms are sore after many days of working with the ceiling.

I wish I had the option of trial and error once the cabs were up, but I can't afford to have drywall compound everywhere once the cabs are installed plus I can't get into the 5" opening (space between top of upper cabinet and ceiling) to do repairs should the finish not be "good enough.

Back to my original question,

""How perfect does the drywall repair have to be if I'm installing 5" crown molding but leaving it approximately 1" from the ceiling. In other words, how far back will you see in a 8' ceiling kitchen?

It depends on how far back you can stand to look at it because the angle will become less acute. You will be able to see further into the corner the farther across the room you can stand.

Draw it to scale on graph paper and see what I mean. Use 5'6'' as a typical eye level.

I would probably not put lighting behind it, this will accentuate any imperfections and possible cause some shadowing that indicates imperfections that you can't see otherwise.

My question is, how thick is the crown at the top? If it's got that 45 degree profile where a 5" crown sticks out like 3.5" at the top, no one is gonna see it, even if they got a ladder.

I just want to follow this post as I want to use crown moulding.