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LATEX PAINTS OVER OIL-BASED

Question
Can you paint over an oil-based paint with latex paint or is it vice versa?

Answer
The rule of thumb is that, given proper surface preparation, for exterior use you can apply quality latex paints over oil-based, but not the reverse. However, if you have many layers of oil based paint, stick to using oil on oil. For interior use, generally you can use one over the other. Some manufacturers of latex products will recommend a primer when going over oil-based paint.


Question
Can I paint over oil base exterior paint with a latex paint?

Answer
Yes, you should be able to use a top quality exterior latex paint over oil based paint, if the surface is well prepared. (However, if the old paint consists of a buildup of say 5 or more coats of old oil paint, the safest approach is to continue with oil, because the latex paint can lift the old oil paint if not adhering well.) All dirt, mildew and loose paint should be removed. Sand any glossy areas to eliminate the gloss and get maximum adhesion from the new paint. NOTE: do not sand or otherwise remove any old paint if it may contain lead. Call the EPA at 1-800-424-LEAD for guidance. Some quality exterior latex paint may be applied directly to clean, sound oil-based paint; but check the product directions, as some manufacturers will require a primer, either latex or oil-based.


Question
As a New England church steeple restoration specialist for 30 years, I run into dozens of layers of old paint. Most of the steeples were built in the early to mid 1800's and have a combination of lead, oil, latex (the kind that will not feather out and lay down) and 100% acrylic. What paint system is best suited to these conditions? My reputation has been built on expert surface preparation. I spend two to three weeks with the sandvik scrapers and power sanding equipment (no gouges) preparing these old steeples before any paint products arrive at the job. All surfaces are wiped down with solvent and clean cloths prior to the application of primer. I have been using a quality oil base primer followed by a boiled linseed oil base satin finish. My paint restorations last 10-12 years with this system. I would like to spec a longer lasting system if possible. Any suggestions?

Answer
A key factor is whether the old paint has been removed down to bare wood. In cases where old paint remains, continue with the oil-based system you have been using. (Things to keep in mind to maximize job life:

  1. spread rate should be kept in the range recommended by the manufacturer; spreading primer or paint too thin will compromise stain blocking, mildew resistance, and crack resistance;
  2. joints and caulking should be done properly to keep moisture out, or let it run out, as appropriate; I suggest using a top quality acrylic latex clear caulk, as recommended by your paint supplier; allow caulking to dry at least one day before applying a coating over it.)

In cases where the paint is stripped to bare wood, use a top quality acrylic latex system:

  1. be sure to apply primer within a few days;
  2. use a top quality latex exterior wood primer; your current primer supplier can recommend one; be sure to apply an adequately heavy coat of the primer;
  3. for the finish coat, go with a top of the line exterior acrylic latex flat, satin, semigloss or gloss paint; your current finish coat supplier can recommend these;
  4. with water-based primers and paints, it is essential that application and curing be at appropriate temperatures: application at too low a temperature can compromise film formation.

This means the paint should not be applied when the air temperature is below the stated figure for the product, nor if the temperature is forecasted to drop below that minimum during the next 36 hours. Also, the surface being painted should not be below that temperature. What can happen is that the lower temperature makes the particles of binder get so hard that they won't fuse sufficiently into a tough, continuous film. If this happens, while the paint may look fine, it may crack or peel or otherwise loose adhesion in a relatively short time, say a year or two rather than say in 15 years.

In extreme cases, the paint will be cracked and/or easily chipped off after it has dried. Color can be affected in that they paint may dry to a lighter color than it is supposed to. Low temperature application can also lead to surfactant leaching, which is described in the problem solver section of our web site. And application of water-based coatings under conditions that lead to too fast a dry can also compromise film formation and thus reduce durability. Thus, avoid priming or painting in direct sunshine and when it is dry and breezy or is very hot (over 95 degrees F).

 

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