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DeborahB
Minimum temperature for exterior varnishing job

Is there a temperature below which it is not recommended to apply urethane on exterior woodwork (door?)

The temperature here is in the mid-50s with about 50% humidity. Obviously it goes much lower overnight, into the 30s.

The can gives drying times for optimal conditions, with 77 degrees. It does not state at which point you'll have the urethane taking too long or simply not drying.

I am using Helmsman Spar Urethane.

My experience is that it dries really fast, normally.

ed21
Re: Minimum temperature for exterior varnishing job

Try to apply it while the temperature is still going up. That way the varnish should be able to set up at least before their is any chance of dew forming on it.
In the 50's might work if it doesn't get too cold and it has 3 or 4 hours at least to set up, but I'd feel better doing it if the temps at least got closer or in the 60's.

ordjen
Re: Minimum temperature for exterior varnishing job

Deborah,

It is never good to use oil based paints and varnishes at too cool a temperature. Coolness and humidity greatly slow down the drying time. In a worse case scenario humidity can also cause "blushing", a condition where the surface takes on a white haze due to moisture being trapped in the varnish film. Also, cool oil paints and varnishes do not flow out, leaving a ropey brush texture.

I am originally from the Chicago area where temps get really cool and damp this time of year. Fortunately, most homes there had storm doors. The solution was to open the door into the house's warmth and lower humidity. On houses where there was no storm door, I would buy an inexpensive sheet of foam insulation board and cut it to size and wedge it into the door jamb where a storm door would be mounted. If you varnish early in the day, you should be able to close the door by late that evening.

You want the door surface to be warm when it is painted. Also, you can warm the varnish itself by placing it in a can of hot water. All oil base paints become more viscose when warm and flow out much better. It was an old painters trick when lacking paint thinner on the job to simple heat up the paint. Years ago, according to my painter father, they used a type of double boiler that had a little heater underneath to keep the paint warm and flowing.

DeborahB
Re: Minimum temperature for exterior varnishing job

Minwax recommended a minimum of 55 degrees.

Which is pretty much what you guys have been saying. Thanks so much for the speedy responses.

And for the great tip for heating up the varnish in a bowl of hot water! What a nifty trick!

Thanks again!

J Roper
Re: Minimum temperature for exterior varnishing job

My suggestion would be to start after about 10 am so the air has begun to warm up. Do not apply the spar varnish in direct sunlight, if you have a choice. Also thinning the varnish a bit will allow it to dry faster but will allow it to flow on smoother. Use paint thinner or mineral spirits. Use just enough so the brush does not feel sticky against the door's surface as you brush. Wait at least 24 hours before recoating. Sand the surface lightly with 220 grit sand paper before recoating and wipe the door down with a tack cloth or a rag dampened with paint thinner. Two coats should give you a good looking finish (3 is even better) and this process will work well at temps. in the 50's.

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