Water Level Controls and Devices. A water column is used on a steam boiler to reduce the turbulence and fluctuation of the water level so the gage glass can provide a steady, accurate water level reading. The use of a water column is not mandatory.
Preparation. Verify the paint is completely dry. Never use varnish to seal acrylic paint if it is even slightly wet. Dust the surface of the dry paint gently with a clean cotton rag or paper towel. Rinse a jar or cup with water to remove any dust particles. Read the label on your varnish to find the correct water-to-varnish ratio. Mix the varnish with water in the jar or cup.
You do need to seal the plaster otherwise the new paint won't stick as you point out. My first choice would be to wipe down and then paint with a coat of diluted emulsion as you suggest. You might want to try this on a section of the wall before doing the whole lot to see if you get a good finish.Dilute PVA will definitely seal the plaster and I can't think of an obvious reason why you shouldn't use the wood glue-but there may be differences in the formulation you need to look out for.ou might want to try this on a section of the wall before doing the whole lot to see if you get a good finish. Dilute PVA will definitely seal the plaster and I can't think of an obvious reason why you shouldn't use the wood glue-but there may be differences in the formulation you need to look out for.