We did extensive R&D with horsehair plaster last year; we re-created lime plaster walls in a room of an 1817 house we were restoring, and used it for repairs in other rooms. We were able to buy materials from Virginia Lime Works, and apply three coat work over original hand-split lath. There is a learning curve, but the finished product really has the exact look, and substance of the original process.
When you say hairline cracks, I wonder what you mean. It is typical for the white coat to develop spiderweb cracks on the surface that pose no problems or threats; if you mean large settling-type cracks (fissures) that originate at the lath, then those have to be addressed.
Drywall materials are not suited for lime plaster repairs, especially not in a historic restoration setting.
Beware of using any patching product that contains gypsum for skim-coat repairs, as it is not chemically compatible with lime-based plasters. If your plasterer wants to use them, he's running a risk of premature or rapid failures.
The huge downside to lime plaster is the very long cure period, which precludes any paint application for at least 90 days on new walls (which is why gypsum became the standard; speed over quality).
Lime plaster has great flexibility which is why it's lasted on the walls as long as is has done.
Is your house brick or frame?
Casey

The house is brick. The east side of the dining room, however has a wooden bump out with three windows in a sort of "bay window" arrangement. The main ceiling is 9 ft, but the extension is 7'6". The south wall used to be an exterior brick wall, but the porch was enclosed about 100 years ago. The brick is now covered by the wainscoating with plaster above that.

When we moved in 11 years ago, we didn't touch the dining room. It was the rental apartment and the wallpaper and wainscoating were in good enough shape. Now that this will become our DR, my wife wants the wallpaper gone.

Most of the plaster is in good shape, it's just a little rough for painting. There are two or three significant cracks. The biggest is a triangle about 4" at the base of east wall where the bump out is.

As I mentioned above, we had all of the walls skim coated 11 years ago and then we painted them. So far, there are no signs of any cracking or peeling of the paint or plaster. In some cases, with the renovation we moved doorways, so we cut into the skim coated plaster walls. There were no issues with the skim coat separating from the horse hair plaster. Is 11 years not long enough to see the effects of the incompatibility betwen the skim coat and the plaster?

Thanks for the help.

The usual problem with applying drywall mud is the use of pre-mixed mud that hardens by drying out.

It can actually dry so fast it forms a poor bond.

If the mud has stayed put for 11 years the plaster may have been rough enough to establish a good bond.

While setting drywall compounds work well for lime-plaster repair, they have even more issues with premature drying.
These compounds do not harden by drying but by chemical reaction.

If the moisture is pulled out of them by the plaster they touch quickly they revert back to powder to some degree.

Bonding compounds can prevent this, or just misting the plaster with a garden sprayer (or hand sprayer for smaller repairs) until the water barely stays on the surface.

At that point wipe away ant visible drops of water and apply the mixed setting compound.
It should be mixed about as thick as peanut butter (smooth please). The minimal water reduces shrinkage cracking as the compound dries.

Hydrated lime and gauging plaster are the origginal surface, and are still available.

The problem is that thelime powder really needs to be mixed and aged for 5-7 days before adding the gauging plaster and additional water to get a workable product.

In England they used straight lime putty plaster, but its setting time is months.
By adding gauging plaster the setting time is reduced to hours, but it still needs to dry completely before any further finish is applied.

I have currently researched and tried a couple solutions to this. I have found that the solutions given by professionals are wide ranging and very contradictory. First, be sure the answer is for OLD HORSEHAIR plaster. Many solutions are for plaster, which is different

The options and contradictions given:

1. Use premixed joint compound. My friend did this totally successfully throughout her home 15 years ago- giving a skim coat of about 1/16" and sanding it smooth. And several professionals (at paint stores and on-line) have said this is the way to go. BUT, many contractors say NOT to do this because the plaster, if it is lime based, will resist the gypsum in the premixed drywall plaster and it will cause it to prematurely (within weeks or years) fail. I used it on one wall, and it dried nicely, but was not as solid as plaster or spackling compound. It primed and painted well, though, and the paint adhered well. So I am not convinced this is not a good solution. But time has not tested its permanence.
2. Use setting joint compound. I used this after talking to the people at USG about their products, and they said NOT to use the premixed, but to use the setting type. That it would adhere. I scraped it on thin onto one wall, but, as the person here suggested, I saw that it dried quickly, possibly before it had a chance toset. Then it would not be hard.. just a dried powder. Went back to USG and he said it needed to be at least 3/32” (who can measure that??) thick to insure it has time to set. I did the rest of the walls thicker. The next day, the walls were very patchy of darker and lighter grey. I went to wet sand, but the stuff just washed off. It seemed dry, and and about as hard/soft as the premixed, so I let it go.. but was concerned. The next day I primed it with 123 Zinsser primer. Two days later, put the gentlest tape on it to hold up a paint chip… and the paint peeled right off. Went back to the bag of compound, and noted that it said the product would get HOT when setting. My walls never got hot… or even warm. On my last batch, the plaster did not even set up in the bucket after 2 days.. it was still cool and moist. So I do not think it ever set. So I have primer over a layer of dust in a very difficult-to -paint stairwell.
3. Use Veneer plaster or Plaster of Paris. This I have not done. When I first started out a contractor said I should not use any type of joint compound, but this would be a harder, better surface. I bought the bag and was about to start, but then read that I would need to let it set for nearly 30 days before I could paint. Also, many sites say you need to use a plaster bonder between the two plasters. I found that the plaster bonder was not sold by anyone locally (I would think if it was so necessary, it would be carried), and it was going to be 2 weeks for me to get it. I suspect the idea solution is to use the plaster bonder, and then the veneer plaster. And go for the long time period. But I went the above routes after researching these other "options". In retrospect, 5 weeks later, I should probably have done this.
4. Durabond. I did not use this, but it is supposed to adhere to the horsehair plaster well. Only it is super hard to sand afterwards. My neighbor used it once in his house, then took the remainder of the bag to the dump as it was impossible to sand. He went with Plaster of Paris.
5. Spackling compound. This has not been suggested by anyone, but, after having gone through all the above, I think I will use spackling compound. It adheres to everything. Hardens well, Paints well. Is compatible with all surfaces. And since I am only scraping on the thinnest layer, although it is pricier, it is worth it in the long run.

Does anyone know of a reason this would not work? I have 2 more rooms to go.

Hello i would have to disagree with all of the above that im reading here

easy sand durobond 90 is the way to go

if to many cracks on walls then mesh the whole wall usually two coats of durobond and one coat of nicely mixed compound like cream mix

then light sand

wait 24-48 hours to complete dry then prime

if to moister in room then use a coupl of fans to help dry