In order to join a plastered masonry wall to a new plastered stud wall, you must do surface prep and moisture barrier, offset the wire lath from the studs using some combination of sheathing and or furring, plaster, then do a skim coat to blend seamlessly.
You’ll account for any “missing” thickness such as the missing inch in your 6” new / 8” old joint problem by using furring or shims etc to separate the base of the coating away from the wall structure.
It is possible to flare the surface out so that the old and new wall sections can be different thickness along the bulk of their spans.
- Easiest way that comes to mind is to draw the curve and cut out curved plywood strips to stand out from the new wall structure to act as anchors for the wire lath.
- Or if you’re not trying to have a notable curved lip at all, calculate to under 1/32 per inch like it’s shelf deflection allowances, and shim out as you move down the wall.
- Attach anchors/shims to new wall
- Staple a little tighter wire lath to the plywood anchors if it tilts a lot
- Mix and go to town plastering.
BUT BEFORE YOU START...
Are you sure it’s plaster and not stucco?
To be successful, you must be aware of what material you are seeking to blend into. Repair or add to like with like to minimize cracking and separation over time.
There are 3 major categories of spread surface covering.
- Gypsum Plaster
- Lime plaster
- Portland cement stucco
To identify what you have:
- Check photos of each type online, compare, then watch & gather the wasteage as the metal stud wall is mounted.
- You can field test for gypsum by taking the powder from the drill aside and check texture as you add drops of water.
- Gypsum really moves if you give it some motivation.
- Lime pretty much won’t move except in a sandy way after it’s cured.
- Portland cement-based (“traditional”) stucco is sandier. Think peanut buttery grout.
If this is an outdoor wall anywhere that gets regular moisture you should hope that it’s not finished in gypsum plaster. Otherwise be extra diligent about maintaining a water barrier of paint or the like.
Lime plaster is hydrophobic and although it’s not perfect, it will do well as a moisture barrier. If you told me I’m going to be plastering on a long weekend, I would wish I was working in lime plaster. There is a bit of a learning curve to delivering an even texture though, and the drying process is... for some it’s burdensome and for others it’s a labor of love.
If you’ve used grout or mortar before, Portland cement will likely feel the most familiar.
Whatever coating you’re using, mix according to manufacturer’s directions.
- You’ll want to mix the biggest batches that you can apply before they cure.
- Try not to waste too much, but err on the side of bigger batches because especially when you don’t have much mixing practice you really want to avoid high texture variability.
- You’ll probably want a small rotary hand mixer.
- Clean off any dirt and debris on the existing concrete and plaster wall. Hopefully you don’t see exposed concrete, especially at the corners. If there are big chunks missing you’ll need to fill them carefully.
- Wash the original wall. You’ll want to add some moisture during application later, but right now the goal is removing or identifying anything loose etc that could interfere with adhesion.
- Watch how the original wall surface takes water, especially for a few inches on either side of the seam.
- If an area isn’t getting wet like its neighbors, then your covering won’t stick there without help. If you need the new coating to stick here, either clean the surface, or give your skim coat a framework to stick to.
- Prep the whole original wall because you are going to do an end to end skim coat at the end.
Add furring strips to the studs so you can hang metal lath at thickness.
To calculate furring thickness:
- Measure from each side of the studs at intervals matching your furring strip length to the existing finish to get the offset on that side. Yes, the studs should be aligned with the original wall edge but surprises happen.
- Feel free to select furring strip length based on whatever is convenient to acquire, cut, and maneuver into place. This is less risky than drywall since in your application, you’ll be building out a paste over the whole surface.
- Subtract completed sheathing thickness and your chosen material’s ideal application thickness from the total offset to get furring strip thickness.
For example, if this is an outdoor wall in the US, there’s a good chance the old work is probably Portland cement stucco, not plaster.
If you’re adding stucco over the new section, then the per-side application thickness for this calculation is 5/8 inch or 7/8 inch, but not quite 1 inch.
Apply paper or wire over the sheathing
If you’re doing the finish work yourself, do yourself a favor bear in mind which materials you’ll be working with in what climate. Proper match to the application is essential.
Apply your coating according to best practices for the materials, location relative to water sources / concentrators, and climate.
For example, Hunker provides an excellent training on stucco application.
No matter the coating, do a top to bottom skim coat at the end to bring up to final thickness and smooth out the joints.
Going into detail on best practices on all combinations of conditions is out of scope for this answer, but it’s fascinating to see how much of weatherproofing is surface-agnostic. It’s just another layer, you’ll do great.