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dano248 04-05-2009 11:07 AM

Painting 3 season porch
 
I live in Chicago area and need to paint the interior walls of a 3 season porch on a condo I just bought. I need to paint over wood paneling, and heat the porch sometimes in the winter, but the room experiences the swings in ambient temperature consistent with the climate. Suggestions, please. Thanks.

Nestor_Kelebay 04-05-2009 01:13 PM

If it's only the walls you want to paint, you could use ANYTHING BUT primers and paints meant for interior use. So, any exterior latex or oil based primer top coated with any exterior latex or oil based paint will work fine.

So, if you're only painting the walls, then you can treat this situation as exactly the same as painting the outside of your house.

However, if you're also painting a wood floor, then you need to go with the hardest coating that will accomodate the possible swelling and shrinking of the wood, and that would be an exterior alkyd primer top coated with an exterior alkyd paint.

If you decide to go with an alkyd paint, I would opt for a FLAT alkyd paint on the walls. The reason why is that alkyd paints don't have anything that helps them stick to an underlying coat (like latex paints do), so you generally need more prep work painting over an alkyd paint than a latex paint. Typically, it's recommended that you sand down a glossier alkyd paint before repainting. Flat paints are, by their very nature, rougher than other paints, so you can paint over a flat alkyd paint with less prepatory work than a smoother alkyd paint because you don't have to sand that gloss off. Latex paints are a different kettle of fish, and so you only need to roughen the surface of semi-gloss and gloss latex paints before repainting.

You don't need to know the rest:

Thermal expansion due to temperature changes isn't a consideration when it comes to choosing paint for use over wood. That's because the thermal expansion of materials is tiny, and every paint will stretch enough to accomodate thermal expansion due to temperature changes from summer to winter of every material I know of.

What's important is the change in the dimensions of wood due to changes in it's moisture content caused by seasonal changes in both the temperature and relative humidity of the surrounding air. You see, wood continuously absorbs moisture from the surrounding air and evaporates moisture back into the surrounding air. Where the temperature is relatively uniform year-round (as in wood furniture indoors), the equilibrium moisture content will depend just on the relative humidity of the indoor air (which changes substantially from summer to winter). For wood outdoors, the equilibrium moisture content of wood depends both on the temperature and relative humidity of the surrounding air, and that equilibrium moisture content of the wood can easily vary from 10 to 25 percent from summer to winter.

When you cut down a tree, initially the wood cells are full of water. As the water evaporates from the wood, the water inside the cells is lost first. Once the wood moisture content goes below about 30 percent, all the water inside the cells is gone, and the water inside the cell WALLS starts to evaporate. As that water in the cell walls evaporates, the THICKNESS of those cell walls starts to decrease, and the wood starts to change dimensionally. Because wood cells are shaped like long drinking straws with their ends closed off, the shrinkage across the grain of the wood is very much larger than the shrinkage along the grain (cuz of the relatively larger number of cell walls across the grain). Similarily, if dry wood gets wet again, the water is first absorbed into the cell walls and the wood swells up again. Below a moisture content of about 30 percent, the amount of shrinkage of the wood varies directly with the wood's moisture content, and in soft woods, you can have up to 8 percent shrinkage across the wood grain from a fully saturated condition (cell walls full of water) to an oven dry condition. That's a full inch of shrinkage in a 2X12!

Oil based primers and paints simply don't have the elasticity to accomodate such swelling of the wood, and so the primary difference between interior and exterior alkyd paints is that exterior alkyd paints are formulated to dry to a softer film that will stretch to accomodate wood swelling, whereas interior alkyd paints dry to a harder film that won't stretch nearly as far. Interior alkyd paints will simply let go of the wood if it swells too far, and the result will be that interior alkyd paints will crack and peel prematurely on wood outdoors.

Equilibrium Moisture Content

Wood Equilibrium Moisture Content Table And Calculator

Latex paints are inherantly softer than alkyd paints, and so BOTH interior and exterior latex paints are soft enough to accomodate wood swelling. The primary difference between interior and exterior latex paints is that exterior latex paints will have a lot more mildewcide and UV blockers in them. Also, exterior latex paints will often use coalescing solvents that have a stronger smell, and will often use zinc oxide instead of titanium dioxide as the white pigment. (That's cuz titanium dioxide will promote chaulking under intense sunlight and zinc oxide is a natural mildewcide.)

So, based on the above explanations, if you're wanting to paint over concrete or metal outdoors, you can use an interior alkyd paint if mildew isn't a problem and you live in a more northern latitude where UV exposure isn't really a problem. You can also use interior latex paints outdoors over concrete but they're permeable so they're not a good idea to use over steel or iron outdoors.

You can learn more than you need or want to know about latex paints at:

http://www.paintquality.com

That web site is maintained by the Paint Quality Institute, which was established by and is funded by the Rohm & Haas Company, who are the largest manufacturer of the polymethyl methacrylate (also called "Plexiglas") resins used to make latex paints in North America.


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