Peel-Stop Triple Thick Primer, by Zinsser
How thick is 30 mils per coat? Well, it'll hide scaly, alligatored finishes without you having to scrape and sand, a serious relief if you're facing a piece covered in lead paint. It dries opaque white, like a primer should, and plays nice with both latex and alkyd paints.
About $35 per gallon; rustoleum.com