Primer for Paint

Visit http://www.askthehardwareguy.com for more tips.
I show my favorite primer for paint. This is the product I suggest to my customers at the hardware store for most painting projects.

Zinsser Bulls Eye 123 is an all surface all purpose primer.  This water based primer dries quickly and can be painted over in just one hour. 

This primer works inside or outside.   It will stick to anything from drywall, wood, masonry and metal, to hard to stick to surfaces such as glossy paint, fiberglass, PVC, formica and even glass & tile.
Bulls Eye 123 is an amazing product that sticks without sanding the surface first (as you should do before applying most other primers).

For water stains, you should use oil based or shellac primer.  But for everything else, it's Bulls Eye 123.

You can use it in high humidity areas such as bathrooms or laundry rooms. 
 It is mold and mildew resistant.   It will also block out stains from crayons, graffiti, grease and tannins (the pigments that are responsible for a plant's color).

 For chalky surfaces like aluminum or previously painted siding, it will prime and seal the surface.   Bulls Eye 123 also inhibits rust on clean metal, eliminating a rust inhibitive primer.
You can also tint Bulls Eye 123. The best color to tint a primer is grey, no matter what color the top coat will be. Grey tricks the eye to blend with the top coat.

I suggest a primer for all painting projects to even out and fill the pores of the material being painted.   This gives a uniform appearance for the color and sheen of the finish coat.  
Primers are designed to adhere better to a variety of surfaces than paint.  Primers give paint a uniform easy to grab surface to stick to.