First off, kudos for having the courage to tackle this ceiling plaster repair job yourself, Janine. We love to see homeowners doing repairs the right way and making the effort to preserve historic plaster.
Thanks to gravity, ceilings present a unique problem when it comes to conditioning the lath and plaster in preparation for the adhesive. Indeed, the liquid will want to drip back out. I have discovered one tool that helps, though — a pressurized garden sprayer, which you can pump as much or as little as necessary to produce a stronger stream than the standard spray bottle.
You can purchase these sprayers at any home improvement center. The stronger stream will penetrate deeper into the lath-work, resulting in less back-drip. Additionally, I’ve found that simple clay plugs inserted into the holes keep the conditioner in (just make sure you remove the plugs before injecting the adhesive behind the plaster).
Also, make sure you go around the perimeter of each hole and glue the plaster at every lath. This will stabilize the open edge of the old plaster, keeping it secure.
Send us some photos of your progress to firstname.lastname@example.org so we can share your work with the world.