Originally Posted by amakarevic
i have made significant indoor plaster repairs (as i calculated it was simpler and cheaper than framing + DW) and now i need to paint it.
the question is: what should i do to prep it for the paint? is regular primer OK or should i use something else (or in addition and in which order). i have heard some emulsion is needed but i am not very familiar.
Hmmm...you're probably not aware of it, but you bring up a plethora of issues that just don't get addressed all that often anymore. Plaster, unfortunately, is not a common building material anymore (at least in most areas of the U.S.) due to the labor-intensive expense of application - plus all the other eccentricities and behavior characteristics of a substrate that can sometimes be "difficult" to hold paint over long periods of time.
To begin, plaster is a highly alkaline, masonry/cementitious coating that will require 30 days to cure before priming/painting. When plaster was a more common building material, the only primer recommended was an oil based product since latexes (of that time) were pretty much guaranteed to burn and peel shortly after application. Back in that day, painters/craftsmen used to size new plaster with a product called "Adhesium" glue size - a clear wall size that, when applied to fresh plaster, would turn pink if the plaster was still too hot to paint. This told the painter that either more cure time was necessary - or more prep was needed before applying oil based prime coat (while the "Adhesium" trade name is owned and used by the Muralo Company, I don't think the original glue size is even made anymore).
Since that time, paints have changed dramatically and the recommended products have pretty much done a 180 degree role reversal. Oil based paints (back then) were more oil - and much less alkyd than they are today. Modern day "oil based paints" are actually more alkyd and less oil, and don't work particularly well on plaster if there is any
degree of moisture present (which there always is with plaster - remember, it is a cementitous surface containing soluble salts that hold water)...Alkyds applied to an alkaline surface, with moisture present, will result in saponification (making soap) that causes alkyds to peel in sheets.
At the same time, latexes have changed even more dramatically. Back then, latex paints were pretty lousy products. They had a hard enough time adhering to innocuous, inert surfaces - let alone the more "difficult" surfaces such as plaster. The "latex" products of today don't even use the same type of resins that were used back then - the only similarities are their ability to dry fast and clean up with soap and water. Todays acrylics have outstanding adhesion, and are pretty alkali resistant (that's not to say surface prep isn't necessary for new plaster)...and today's acrylic primers hold their purchase to new plaster far longer than the old oil based products from many years ago.
So...to finally answer your question, allow your plaster to dry 30 days. After 30 days, your plaster should be hard and relatively slick. I've always believed the best prep for plaster is to scuff sand with medium grit paper, followed by sponging the surface with a mild vinegar solution. Unfortunately, the vinegar solution then needs to be neutralized with clean water and allowed to dry before priming. The vinegar wash neutralizes surface alkalinity - plus slightly profiles the hard plaster for better adhesion of the prime coat. A vinegar etch may be unnecessary, but it's easily applied and may actually prevent problems down the road.
There are many brands of acrylic primers that can be used for new plaster (and there are other types of primers beside acrylics that will work) - Zinsser makes BullsEye 123 (acrylic - my recommendation), or Cover Stain (modified oil), or BIN (pigmented shellac)...each would work, and each can be found in most paint stores or hardwares. Talk to your local independent dealer for his recommendation of product brand that'd work best in your application - and good luck.