I accept the need to prime unpainted surfaces before painting, but I have a couple questions about the use of primer:
What is so special about paint designed to be primer? It’s not that much cheaper than a gallon of flat white, and don’t all paints seal against moisture nowadays? Does the primer change depending on the surface (wood vs. metal vs. drywall)?
I’ve always thought there was no reason to prime an already-painted wall. Now I see manufacturers recommending use of tinted primers if you’re painting a dark color over a light one. Is this just a way to sell more paint, or is there really a point to this?
Painting a surface directly can have adverse effects. You may not get an even coat, as the surface absorbs more paint in certain areas, or the surface may not be smooth in certain areas. Primer fills in the gaps, makes sure the surface is nice and smooth, and gives the paint something to stick to. It is also more protection for the surface and increases the life of your paint job.
I can see why you would want to prime a dark surface if you are painting a light color over it; the dark will be hard to cover. Paint will stick better to primer than another coat of paint as well. Perhaps the opposite could be true with the colors: if you’re paint job were nicked it would both be easier to repaint if there were primer underneath and if the primer were of a color closer to your paint it will be less noticeable and likely to show through. And perhaps they ramp up these features to sell more buckets.
The main job of paint is to cover a surface with color. The main job of primer is to stick to the surface and let the paint stick to it.
As a matter of practice you can get away without using primer in a lot of cases, but usually you’re better off priming first. To a degree it depends on what your standard of quality is.
Yes. Certain primers are specifically designed for metals, wood, masonry, etc. For the most part any primer will work OK on any surface. Again it depends on your standard of quality.
Depends on what the wall is painted with. Painting latex over a glossy oil based paint absolutely requires a primer, but latex over flat latex is OK without priming.
My experience with tinted primers is that they don’t help much. Primers have enough white colorant in them that it’s hard to tint them past a pastel, and that really doesn’t provide any extra coverage that I can notice.