I am patching some plaster with some drywall and have shimmed out the drywall. The issue is that the plaster isn't a consistent thickness. Some spots it's flush and others its off by 1/8". What's the right course of action to level these out? Paper or fiberglass tape with that big of a difference would make the bulge worse. Heard of a product called fibafuse that might be good for this? Recommendations?
With uneven work like that you will have to build up the sheetrock side I use a wide mud knife at least 12" when trying to cover uneven areas, as far as the kind of tape paper or fiberglass I use both but preferr fiberglass in this case as it helps reduce cracking. Also I would not do in 1 coat but use several thinner layers. For areas like this I will use "hot mud" because it sets quickly and in some cases as soon as the first layer is complete the starting area is dry just don't mix two much up or you may have a mud tray that turns into a brick before you get done. The last caution is hot mud is tough to sand so I will put my base coats on then a thin layer of topping mud very easy to sand but it takes a day to dry.
I would sand down the plaster side, about 6-8 inches in from the joint, then add tape and feather out as normal. There really is no other way to mask the rise that would be seen from adding taping compound over the existing plaster. Save for adding a ridiculous amount of compound to the new drywall.