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OIL OR LATEX

Question
My question on paints is how do you tell what type of paint is on a surface. If paint is peeling, how do I find out if its oil or latex paint? If its brittle and brakes when you crunch, is this oil?

Answer
If the paint is at least several years old and is peeling, take a piece of it and bend it. If it cracks, it is most likely oil based or alkyd paint. If it seems flexible, it is probably latex. (However, highly pigmented economy latex paints can tend to crack when flexed this way.) If it is a combination of one type applied over the other, then the oil-based portion is more likely to crack, while the latex part would be more flexible. Another test would be to soak the piece of peeling paint in some acetone or fingernail polish remover. If it dissolves or significantly softens, then the peeled paint is likely to be latex. Be sure to use safe practices when working with acetone or fingernail polish. Protect your skin.


Question
For exterior painting, how do you decide whether to use latex or oil based paint? We have hot, humid summers and moderately mild winters.

Answer
You would use oil-based paint if you have a build-up of old oil-based paint on the surface, say more than 3 or 4 coats. Also, if you have to apply the paint when it is below 50 degrees F, use oil-based paint. Also, if the surface is chalky and you for some reason cannot clean it of chalk, use oil-based paint. Otherwise, a top-of-the-line 100% acrylic latex paint will be suitable, and if done right will give you longer crack resistance, better color retention (especially where it is sunny on your home), and better resistance to mildew. Be sure to follow the "Steps to Success", which you can access from our home page at www.paintquality.com


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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