Great question — answering as the UphillTech team:
If the existing paint is well-adhered to the wall and does not have blobs that need to be sanded down to smoothness, this sounds like a job for shellac based primer without any other prep. Repair any holes or divots, of course, and sand down any high spots, but I think you should skip the washing and general sanding recommended by others. Shellac based primer covers a multitude of sins. It's very thin and penetrates porous or absorptive surfaces, so it should consolidate any chalky areas into a solid, smooth surface. I would use two liberal coats applied with a short-nap roller. It's composed of shellac dissolved in alcohol with a small amount of white pigment. Be aware that the alcohol...
Full answer by MTA: https://diy.stackexchange.com/a/315631 (CC BY-SA 4.0).
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