Exterior Latex Paint won't dry/cure

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Old 06-07-11, 04:59 PM
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Exterior Latex Paint won't dry/cure

My husband just built a set of Adirondack chairs using pine lumber. The lumber was completely dry and the humidity outside was very low, around 30%. Outside temps were in the 90's. All doors and windows in his shop were open and there was good air circulation. He primed them with latex primer and then painted them with 2 coats of Valspar exterior satin. They have been painted for approximately 3 weeks and are still tacky and soft. Is this normal? If not, is there anything we can do to make them cure without having to strip all this paint off and repaint them?
 
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Old 06-07-11, 09:15 PM
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Was the Valspar exterior satin an oil based or latex paint?

What colour was the Valspar exterior satin?

I'd take a hair dryer to one part of one chair and see if that accelerates the drying.

It would have been much better to use an oil based primer and an exterior oil based paint on these chairs. Latex paints dry to a soft film that doesn't stand up to wear and tear. That's OK for a wall or ceiling, but when it comes to a working surface, like a table top, floor, window sill, shelf or even a chair, you really should use an oil based paint.

Interior and Exterior latex paints dry to about the same hardness. Typically, interior latex paints will have resins in them that form harder films because interior wall paints have to stand up to scrubbing to clean off stubborn marks. Exterior oil based paints dry considerably harder than latex paints. Interior oil based paints dry harder than exterior oil based paints. That's because exterior oil based paints need to stretch and shrink with wood outdoors, and with higher hardness you lose elasticity and flexibility. So, exterior oil based paints are formulated so that they don't cure to as hard a film so that they have sufficient flexibility and elasticity to stretch and shrink with wood outdoors as it swells and shrinks with changes in it's moisture content caused by seasonal changes in temperature and relative humidity.

"Oil based" polyurethanes dry to the hardest user friendly coating. Oil based polyurethanes are really nothing more than alkyd (or "oil based" paint) resins with urethane groups right inside the resin. Urethane groups:

-NH-(C=O)-O-

are particularily strong (and I don't know why), and they act very much like the roll cage inside a race car, making the resin harder if you tried to squeeze it or stronger if you tried to stretch it. This is why polyurethane varnishes behave so similarily in so many respects to clear oil based paint.
 
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Old 06-08-11, 03:43 AM
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How long did he wait between coats?

I don't know a whole lot about the valspar line of paints but often the cheaper grades of latex enamels won't dry to a hard finish. It will feel dry to the touch but if you use pressure - it feels soft. Does that describe your paint job? You might try contacting Valspar's customer service dept and see what they say.

I agree with Nestor that an oil base paint will dry to a harder finish although they won't always wear as well in regards to the sun and rain.... especially in the south.
 
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