Good thread. I have been using Weld On #4 for years. Many years. Great stuff. I have used Tenex, Weld On #3, and a few others not worth mentioning. I have settled on Weld On #4 because 1) Weld On #3 not that easy to find 2) works with styrene, acrylics, and other plastics 3) cheaper than anything else, especially in bulk. I currently buy it by the pint from a plastic company in Missouri. This quantity last a long time, and I do a fair amount of modeling.
I agree, any of this stuff evaporates at an alarming rate. However, I have discovered two things that seem to effect the evaporation rate: location and quantity.
Location: Weld On evaporated less when I lived in Iowa and Arkansas- possibly colder and less humidity. Lived in Florida for 11 years, seemed to evaporate quicker there- yes, you would go the the local hobby shop and their stock would have half filled jars. One store finally stopped carrying the stuff due to the evaporation. Others only ordered on demand. And what they got from their vendors was missing quantity.
Quantity: As I mentioned, I buy it by the pint. A full can seems to evaporate at a much slower rate, even though I leave it open for several hours at a time, than when the can gets down to 1\8 full. When it reaches 1\8 or lower, you can almost watch it disappear.
So, that's my experience- hope this helps.