Like paint, primers are available in both oil-based and water-based formulas. Water-based primers are best used on bare drywall. \
Oil-based primer is the best way to prepare oil-based paint to be painted over with water-based paint. The primer will cover the old paint and give the new paint something to stick to. \
Water-based primer will not stick to oil-based paint as well. If you're determined to use a water-based primer under the water-based paint, do a coat of oil-based primer and then do a coat of water-based primer before moving ahead with your painting project. \