I recently used the water base stuff on the underside of two fenders. One fender was near a heater and was just warm to the touch. The other was at the other end of the work bench and cold. The ambient air temp was about 50 degrees. The warm fender turned black in about 15 minutes. The cold fender still has a white milky look. I think the temp is an important factor. The directions said that it needs to be above ??? temp, which I can't remember at the moment. Apparently, they mean it!