It is a common myth that primers are designed to cover your old paint so that you can paint a different color easier. The truth is primers are not designed to dark or intense colors. There are two exceptions however. They are: PVA or drywall primers which serve several purposes, one is to provide high- hide over new drywall and colored primers that are used when using intense colors such as blue-based reds. It can take up to six coats to hide a previously white wall. These tint-able high-hide primers can be tinted to a shade close to your finish paint. This will allow you to get away with only painting two or maybe three coats. Remember - any primer can be tinted to a shade similar to your finish coats. The paint store can do this for you. Primer comes in white, but it doesn't need to stay that way.
However, if you have a dark red room that you want to paint a lighter color, don't bother with painting a coat of primer first to hide it. That would be a waste of time and money. Most primers are low-hiding because they are not designed to hide. Instead, do two coats of your lighter color. You will end up putting on two coats of your new color regardless of whether or not you prime over the old color or not.
Primers are designed to provide excellent adhesion and bonding, to seal water soluble stains like water stains, knots, magic marker etc., and inhibit or stabilize rust. Usually painting fresh latex paint over old latex painted walls needs no primer but painting over weathered, glossy surfaces or oil based paint priming is a really important step.
Products like KILZ and BIN provide great adhesion and stain-sealing. You should never paint over unpainted, lacquered woodwork or oil or alkyd paints without using a primer of this type. If you do, your finished job will peel and scratch easily, and there really is no solution but to remove all the paint back down to the original finish and start over again. If not, you will need to “touch up” frequently.
PVA or drywall primers are not very expensive and are high hiding. They provide tight bonds to new drywall, and a fairly opaque surface for your finish coat. The best way to tell whether drywall was primed before painting is to firmly press a piece of scotch tape to the wall. If it was never primed, you will probably yank off all the paint right down to the drywall when you take off the tape.
All new wood, interior or exterior should be primed. This serves several functions, including inhibiting tannin and knot stains, preventing rot, and ensuring that the finish coats will be more weather and scrub proof. If you are dealing with exterior trim or siding that has been scraped down to bare wood, use a high quality breathable primer over bare and weathered wood. Doing this will easily double the life of the paint job.
Before repainting the outside of your home, go to the side of your house that gets the most sun light (usually the south or west side). Run your hand over the siding. If a chalky residue comes off onto your skin, you must thoroughly power wash and scrub the house, or prime it with an oil based primer. Power washing will be much easier. Some exterior paints are described as "self cleaning". These paints are designed to wear off over the years, along with soot and discoloration. Even the best exterior paint will not adhere to this surface and will be peeling everywhere within a year.
There are basically two types of metal primer. Most good metal primers inhibit rust. To rectify a flagpole or ironwork that is too severely rusted to clean down to the bare metal, there are rust stabilizers available that once painted on the corroded metal, will turn the rust black as well as inert. The surface can then be painted within an hour.
There are many other primer types that perform various functions from sealing and waterproofing cinderblock, zinc primers for steel, epoxy concrete sealers, and so forth. Generally, these are not used much on residential homes; if you have any questions, check with your local paint store.