You must always prime new surfaces!!! Without using a top quality primer as a first coat, the finished application does not stand a chance of adhering properly to the substrate in which you are painting. Furthermore, the binder in the finish paint soaks in and the result is that too little binder remains for the pigment to adhere with. This conditions results in uneven gloss, surfaces that get dirty faster, and poor scrubbability.

Remember that depending on the surface you are painting dictates what primer you use . For example, you would not use a metal primer for interior wood sash. Some benefits would be derived from the sealing ingredients in the primer but metal primers contain zinc that when oxidized will adhere directly to metal and steel. Wood primers on the other hand are geared for absorption's into the wood fiber. The better wood  primers contain stain inhibitors that prevent tannins from rearing their ugly faces at the surface. BEWARE!!! Many paint companies tout the world about their particular primers claiming they cure everything from cedar bleeds to Kleinfelters disease. A good example if this is the legendary primer Bulls Eye 1-2-3 by the Zinseer Company. This is an acrylic primer (water based) that claims a plethora of attributes. High on the list is the prevention of cedar and redwood staining . When I was a neophyte in this business if I saw it in writing I assumed it was true. Like most things in life you live and learn inevitably at a cost. This way the case with a new addition we were doing on a home sided in new cedar shingles.    I primed the new shakes with 2 applications of Bulls Eye 1-2-3 primer and to my chagrin the monster of cedar bleed surfaced sporadically about.  Disgruntled, I called the Zinseer company and like most vendors I've dealt with  they "passed the buck".  The Zinseer technician said maybe the wood had a moisture content above a certain level and that the lumber company may be to blame.  Calling the lumber company was even more frustrating.  They said I should have used an oil-based primer which would have scaled the tannins and absorbed better than the acrylic primer.    The time and the material to correct this and with no guarantee of the results made for a difficult decision on my part.  Because my reputation was on the line I opted to find a top quality oil-based primer.  I used Benjamin Moore's Moorwood primer which is an alkyd primer (oil-based), and low and behold this eliminated the bleed.   It did so because technically the binders in an oil based primer have better penetrating qualities and volume of solids which hold back tannic acid bleeds.   The down side to oil products in general is their lack of elasticity and breathability - in essence you are left with a brittle surface that over time will crack.   In turn top coats lose their ability to adhere.  The time table for failure will vary with the substrate, temperature at application, product used, environmental factors, etc.

The moral of this story is that if you are a do-it-your-selfer try not to believe everything you read or hear.  This is not to say that the product you read about will be ineffective.  Just be careful!!  Call contractors and see what they used in a particular situation, chances are they have lived, learned, and hashed out the bugs for you!!  Let me make something perfectly clear - Bulls Eye 1-2-3 is not a terrible product - it has performed well in many other substrate situations.  Heavy cedar bleed was just not one of them.  It takes experience and time to learn all the products and their applications.