Oil or Latex primer?


  #1  
Old 09-28-00, 07:26 AM
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I will be painting some red cedar trim on the exterior of my house and was wondering if I should use oil or latex primer on the bare wood. I have read previous posts that say to use oil primer and I was wondering if it is inferior. Also, can you apply latex paint over oil primer?
 
  #2  
Old 09-28-00, 09:50 AM
mikejmerritt
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RayM, You had better use oil primer on your cedar of any kind because cedar is a known bleeder. I am refering to knots and sap. If you use latex primer of any sort you can look for the knots to bleed in short order. Even with oil the sap will drain from cedar in places at times and there is nothing to be done but remove and touch up. Not to alarm you but it happens often. I can tell you that to prime raw wood with anything but oil is the inferior method. Latex primer will bond to the wood most of the time but has no stainkilling properties. Latex over oil primer is the way to do it.....Mike
 
  #3  
Old 09-28-00, 11:19 PM
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RayM:

This is generally the decision making process I go through when deciding whether to use oil or latex primer:

Oil base primers penetrate better into bare wood, just like stains penetrate into the surface of the wood. You can get better penetrating if you thin the primer with paint thinner for the first coat.

However, oil based primers don't "breathe" well. Moisture in the wood can't pass through the primer easily, and if the wood gets wet because of condensation on a window dripping down into a corner joint or whatever, then the evaporation of that water from the wood will peel the primer off the sill.

So, it's just a matter of deciding which characteristic is more important in the wood I'm painting... the good penetration of the primer into the wood so that it adheres real good or the ability of the primer to allow water molecules to pass through it without lifting it off the wood.

The same goes for the paint. Oil based paint is like a sheet of plastic... it doesn't breathe. All latex paints breathe better than oil based paints, but the flatter the paint the better it breathes. Latex paints won't allow liquid water into the wood, but they will allow water molecules in the air or in the wood to pass through the primer and paint. So, when it's humid outside, the wood gets humid inside, and it subsequently dries with the weather.

Don't know if this helps.
 
 

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