Is It Necessary to Prime Before Painting?

Woman Priming Baseboard Before Painting
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Priming before painting is one of the most vexing questions.  Often, the answer seems less based on objective factors than on subjective ones:  mood, tolerance for painting, and cost.

After all, priming can feel like a wasted effort.  Priming is exactly as much work as laying down the finish color coat.  Every stroke of the brush, every roll of the roller, every linear inch of painter's tape stuck down is the same as you would do for the eventual color coat.

  If you think you can breeze through this stage, think again.

If you have a low tolerance for painting, priming means that by the time you get to the color coat, you've already spent considerable hours handling roller and brush, drop cloth and paint cans.  Your weariness may erode the quality of your finish work.

You May Need to Prime When:

ConditionReason
Surface Is Bare Wood or New DrywallThis is the biggest "yes" of them all.  Newly installed drywall is highly porous in two ways:  the bare facing paper on drywall and the dried joint compound ("mud") covering the seams.  Bare wood is even more porous and always requires primer.
Drywall Surface Is Skim-Coated A skim coat is a thin swipe of drywall compound laid over bare drywall.  Considered a level 5 finish (the highest grade possible), it's not something you encounter often.  But as with bare wood or drywall paper, it is highly porous and thus requires primer.
Previous Coat is GlossyGlossy base coats don't hold paint well.  A light sanding and two coats of primer will help the color coat stick.
Changing From Dark to Light ColorsAvoid the heartache that comes with repeatedly laying down expensive light colored paint over that darker colors.  Instead, first treat it with two layers of white primer; three, if the existing color is extremely dark.
You Are UncertainYou can never go wrong with priming.  If you have little confidence about the condition of the wall prior to painting, the default choice is to prime it.

  

You May Not Need to Prime When:

ConditionReason
Walls Are CleanPrimer tends to stick better to walls in imperfect condition more than will paint.  So, if your walls are perfectly clean, it helps to eliminate the need for primer.
New Coat Matches Previous Coat ColorOne need for primer is to readjust the base color for your new color to brilliantly and accurately show up.  If previous and new colors are the same, once again the need for primer is reduced.
Priming Prevents PaintingThis one is all about psychology.  Are you putting off painting because you don't want to prime?  If painting without priming is the nudge you need to get to work, then do it. 
You Are Using Paint + PrimerSelf-priming paint can be summed up rather succinctly:  it is thicker than regular paint.  Because it is thicker, it builds up higher and forms a thicker coat.  It is preferable to use separate primer and paint.  But if the walls are more or less in good condition, you can use a combination paint and primer.