I just read a blog stating that pre-mixed joint compound should not be put over pre-existing plaster walls. Why? What is the best product to use?
The answer really depends on what you are trying to do. A full skim coat, patching, or covering lead paint. In general, many GP premixed compounds specifically say not to be used as a skim coat.
For the sake of argument, let's assume you are doing a full skim coat. Start with a full bodied joint compound, not a lightweight. Divide the pail in half into another container. Cut it a bit with water and Ivory dish soap, mix extremely well until it is the consistency of pudding. Firm enough to stay on your hawk, but loose enough to glide on the wall. This mix will elimamate most of the air bubbles and give you a very smooth coat without a lot of sanding. Apply with a 12 to 16 trowel or knife. Make your first coat about 1/16" thick. Sand it with a very fine sanding sponge or screen. Give it a second coat to fill the void spots and sand it again. You can wet float it for a glass like finish.