I'm not sure what the established "best practice" is regarding this issue. But, I grew up in a house which was built in the 1880s. When I was young, we lived with the many layers of wallpaper. Later, my parents removed every last bit of wallpaper. Underneath it all was plain, grey, unpainted plaster. They took measures to remove every bit of wallpaper paste (a light sanding was done solely for that purpose.) Aside from that, no skim coat was applied prior to priming, except for where the plaster was not in good condition. The walls look great - there seems to be no need for a skim coat.

That said... The walls are somewhat "wavy". Given that it is an old house, which has settled for 130 years or so, we do not expect perfection, and so the unevenness of the walls is perfectly acceptable. I suppose a skim coat may be added if you desire a more "finished" look.

the idea of the skim coat is to put a very thin layer of compound over the walls to smooth everything out. it may or may not be a good idea depending on the condition of the walls. if it was me, i'd take out the wall paper (hardest part) and then assess the condition of the walls before deciding whether or not to skim coat.

If the plaster is in good shape and flat there is no reason to skim it.

Put on a coat of size and then apply the wallpaper.

I'm not sure if I was clear... my parents painted after priming - they did NOT re-wallpaper. At this point, the walls are all painted with flat wall paint. (I believe the OP is also planning to paint after removing the wallpaper.) If they were to re-wallpaper, the skim coat might be somewhat more necessary in order to create a flat surface. A painted "wavy" wall is fine - a wallpapered "wavy" wall looks odd, especially when there is an obvious pattern in the wallpaper. That was one of the problems with the old wallpaper look, in addition to the wallpaper being a very dated pattern.

Put on a coat of size and then apply the wallpaper.

Size? No, Wall paper primer? Yes

Same thing, different name.

http://billarchibald.com/size.html

Here is a link that might be useful: sizing

Many houses of this vintage were plastered without applying a finish skim coat. The intention seems to have been wallpaper forever. In many cases, when the existing wall paper is removed, you find a rough, sandy plaster that is not suitable for painting unless a smoother, more stable and less absorbent coat is added. Frequently the wall paper removal will also reveal cracks, holes and other defects that must be repaired.

You can hire a plasterer to add an actual plaster finish coat. As an alternative, many times joint compound over wide and thin fiberglass mesh is used. Not an easy job, but unlike actual plaster, within the DIY realm.

We live in an 1820's brick farmhouse in which all walls, including the interior walls, are solid brick with rough coat/finish coat plaster applied directly to the brick. Many of our walls were wallpapered, and there's no evidence of paint under the paper. We are painting all the walls, and have been for years. (long restoration) We do not want to lose the plaster look (imperfections and waviness)but the walls have hairline cracks at best, settlment cracks and missing plaster at worst. All of our walls get a skim coat after we have repaired the cracks/missing plaster, and after we give it a light sanding. Lots of work, but the results are worth it. The look and feel of the old plaster walls has been retained, and they look beautiful painted. We used setting type compound for the skim coat because I think it bonds better and is stronger than the premixed. Harder to sand, but it's a thin coat, and we're careful getting it smooth to start with. Uually only a few trowel ridges to knock down.

"As an alternative, many times joint compound over wide and thin fiberglass mesh is used."

Or skim the walls with Durabond or another setting type compound.

You do not need anything else, just the compound.

Thanks for the response everyone. Think I got what I need. I'll post some before and after pictures when the process is complete!

Much appreciated.

-Luca

Luca, the other thing to keep in mind with your house is the value of the plaster and lath walls vs. sheetrock. If you do some hunting around on this forum, you'll find discussions about it. So I won't cover it all here, but just keep in mind:

The acoustical qualities of plaster and lath are different from sheetrock, and in most if not all people's opinions, better. The varying densities of the component materials (lath, plaster, and sand) and the different makeup of the layers (lath, scratch coat--which has more sand in the mix), and finish coat) really affect sound transfer. When we go to a friend's vacation home, which is very nice but newer sheetrock construction, I can't get over how the whole house is such a rattly sound box.

So, if at some point down the line you are faced with ripping out a wall (to run electrical, put in insulation, whatever), consider that it may be worth the extra time and inconvenience of fishing wires or only cutting out partial sections and patching, than taking out the whole wall and replacing with sheet rock.

They just don't build em like they used to.