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Hi Seth, As I see it you have at least two separate heating issues to deal with:
1. Maintaining a space temperature at or above 60 deg F in your booth for quite a few hours prior to painting so all the heavy castings get up to ambient temperature (since your entire space is unheated). 2. A fairly high ventilation rate needed during painting will bring cold outside air directly into your booth. It is referred to as "make-up" air and it takes a lot of heat energy to heat it up fast enough. You certainly do not want to have any potential sources of ignition inside the booth while you are spraying. An LP heater with a pilot light could be OK for no. 1 but would not be considered safe for no. 2. To give you an idea how much heat energy you would need to provide for no. 2, take your estimated cfm ventilation rate, multiply it by 1.08 and then multiply the result by the temperature rise from minimum outside ambient to at least 60 deg F. For example, if it is zero deg out and your ventilation rate is 3000 cfm, you would need to provide 1.08x3000x60 = 194,000 btu per hour of energy to heat the air up to 60 deg before dumping it into your booth. I don't know where you're at so you might never want to paint when the outside is as low as zero deg. In any event, you can plug in whatever low temp you think is appropriate for you. Some type of heat recovery on the exhaust air out the booth is certainly possible in order to preheat the incoming air and minimize the net heat to be added to the make-up. It becomes more complicated that way though and may be of dubious economics unless you are going to be doing a lot of painting in the cold weather months. Plus it would only be used during the actual time of spraying and for only a relatively short time thereafter. After the initial dry, I suppose you could shut down the ventilation air and go back to an in-booth LP heater to maintain the temp at 60 deg for final drying/curing. I would not consider painting at all if the temperature of either the part(s) to be painted or the ambient air in the booth before, during and after painting is less than 60 deg F. If push came to shove, you likely could trim the "after heat" back to a couple of hours if a hardener is used but I sure wouldn't trim the "before" or the "during".  Rod
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