If you're painting an item with some staining or something with a little damage, a shellac-based primer might be your best bet. If you're painting wood, an oil-based primer is a good choice.

It sticks to whatever you’re preparing to paint and turns it into a smooth ... of course, water (see Shellac: The Original Primer, below, for dealing with severe stains and odors).

Use a sanding block or pad to remove old, flaky paint. Sand in the direction of the woodgrain so that you don’t miss any ...