Stain blockers, sometimes referred to as “stain kill” primers, do a great job in sealing off stains. Some work better than others. The solvent primers form tighter films, and usually work better than their latex counterparts for most stains.
Shellac is the king of stain blockers
The best of the stain blockers is pigmented shellac. It forms a very tight film, and is even a vapor barrier. It will prevent stains from migrating into the finish paint better than the rest, but it is only suitable for interior surfaces, and not all interior surfaces at that. (see Shellac primer).
Oil is probably the next best stain blocker, the original Kilz is such a primer. The biggest problem with the oil stain blockers is the very strong odor they omit. You must ventilate the area well during and after use, or you will be badly affected by the fumes. Pigmented shellac is better than oil in terms of odor, but not much. You must ventilate well with either.
Waterbased stain blockers work on many stains, and are a suitable option, when they can be used. They don’t work well on water stains, or water soluble stains. Their strength is the water clean up, and far, far less odor they omit. Waterbased stain blockers are nevertheless good sealers, where a bleeding stain is not the problem.
How to avoid flashing
First, don’t use a stain blocking primer if you don’t need to block a stain. PVA primers work great for a cheap first coat over properly prepared surfaces, if you need a gray primer (see Painting Deep Tone Colors). PVA primers as a rule are not problematic in terms of flashing. They are my primer of choice over drywall repairs and spackled areas because they are not prone to flashing problems.
Total priming is the antidote to flashing
If you need to use a stain blocker to block out a stain, or a bonding primer (usually these are stain blockers too, but not always), then you will have to prime the area in total, or at least from breaking point to breaking point.
Since two coats of stain blocker is the best prescription for most stains, it is best to spot prime the stain first, then prime the whole wall or ceiling (or room if you have multiple stains). This will serve two purposes: by blocking out the stain AND by evening out the sealed area to prevent any spot flashing.
You can usually use a waterbased stain blocker for the total primer. If you seal the stain in first with either pigmented shellac, or oil based stain blocker (buy a quart for this as needed), you can do the total priming over the top with a latex stain blocking primer / sealer (like 123 Bullseye). This allows you to use a low odor product over the large area, making the total priming less “odiferous“.
You may tint the total prime coat to match the finish paint color (or gray for deep tones) to act as a first coat. Generally, for pastels and off whites, you will use a 50% formula of the finish paint color into the primer to arrive at a pretty good match to the finish paint color. Deep tones will require a shade of gray instead of a color match for good hiding (you will not be able to tint a deep tone from a white primer - anyway). See your paint store clerk for tinting the primer to match to the finish paint color.
Spot priming
When you have a stain, it is best to apply two coats of stain blocker to be sure that the stain will not migrate partially through the first coat. Usually, two coats of the right stain blocker will suffice.
Flashing: the problem with spot priming with stain blockers
It depends on the lighting in the room and where the spot priming will occur, but usually you can’t get away with spot priming with a stain kill primer. The primed spots will look whiter and / or shinier than the rest of the wall/ceiling., this is known as “flashing”.
Spot Priming Stains
with
Kilz or Zinsser Primers
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