A moistening system that significantly reduces the problems caused by contaminants in the moistening fluid is provided. The catch basin of a moistening system is provided with a filter that reduces the amount of contaminants contained in the recycled moistening fluid. Additionally, the moistening fluid is buffered with an acid that acts to dissolve the paper filler and other soluble debris that may be introduced into the moistening fluid, thereby significantly decreasing any blinding of components of the moistening system that would otherwise reduce the flow rate of the moistening fluid through the system. These ensure that sufficient moistening fluid is applied to the envelope flaps, which results in consistent and complete sealing of the envelope flaps.