The main combustion products of any hydrocarbon, including propane, are carbon dioxide and water. The chemical formula for propane is C3H8 - so every molecule of propane that burns produces four molecules of H2O. In other words there's a lot of water in the exhaust from a propane heater. If your workspace cools down a bit, that water will condense all over everything.

Chenier's comments are appropriate and to them I'll add that combustion is seldom 100%, a small percentage of CO typically produced with burning any carbon-based fuel.

Unvented heaters, regardless of fuel type, exhausting combustion byproducts into your workspace, aren't what you want to use. 

   What type of heater would you suggest?  My garage is insulated but not the overhead door, is 400ft2 and I need to raise the temp by about 10 degrees.  I do not have 240 volt service

   Electric heaters are the cleanest but they tend to spin your meter off the wall. An option as we get into spring is to use heat lamps to warm the surface you're working on. A hot box, just a box with a 60w bulb, to keep the epoxy components warm. You could also keep them in the main house if it's close or attached to your shop.

Any kind of combustion in the work area can create soot and raise the humidity which can interfere with the epoxy. Also, carbon monoxide is a danger with any unvented enclosed work area.

Have fun but stay safe.

Not to challenge anyone, but in the quest for knowledge! 

Does a wood stove increase humidity? I've always heard that a woodstove was "dry heat".

There's humidity, and there's relative humidity. I assume it's relative humidity that matters WRT epoxy, but I could be wrong on this.

I know a propane flame releases moisture in the air, and epoxy doesn't like it. 

Is burning wood anything like that? Maybe to a lesser degree?

   Oh, I built a Russell Brown PT Skiff, and erected a plastic tent around it with an electric heater inside, during the winter. Turned it on when necessary. Was nice for me, too!

Sorry about the train of thoughts!

WRT the uninsulated door, you might tack a sheet of plastic spaced a few inches inside and surrounding the door. Sealed as best you can.

My house was right on a wide part of a river, double-garage door facing and wide open to winds from the west. Leaked badly! I braced the door with four 2X4s secured to the floor and header to keep it pressed up to the frame seals.

>> Does a wood stove increase humidity? <<

Wood stoves exhaust their combustion products outdoors through a chimney. So no, they don't increase humidity.

Portable propane heaters, such as we were talking about at the beginning of this thread, exhaust their combustion products into the space being heated. Hence their problem.