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Hi Unruh, Well, it all depends. If you are doing something that is really important then I would start with a fresh supply of hardener. That said, I have successfully used MH168 from a previously opened can that was two years old. No problem noted. The MH167 is a little faster but I would expect a similar result as long as the cap has always been clean and tight. One way that I have heard to extend the useful life of opened hardeners is to fill the air space with an inert gas, such as argon. I have that for welding but have not yet tried it as a blanket on hardeners. There is also a commercial product in a small can that claims to do the same thing but the name escapes me. I believe humidity is a consideration in painting but I don't know enough about the effects to discuss it intelligently. I don't paint in the winter when the humidity is low nor in the summer when it is extremely high. Most likely it's the extremes that you would to be concerned with. I have read that humidity is more of a consideration when using the newer waterbourne formulations of paints. I have never seriously investigated them so I don't know. At the very least, if you don't have very good moisture control (at least with a single stage compressor setup), then high humidity conditions can result in increased water in the compressed air. That's something you don't want mixed in with your paint. Don't confuse the term "isos" with meaning isopropyl alcohol. It doesn't represent that at all. The term "isos" is short slang for isocyanates, a particularly nasty family of chemicals that are common in most hardeners/activators/catalysts used in two part paint products. Fortunately, the MP175 catalyst is one of the few exceptions. It does not contain any of the isocyanate chemicals. That is why a cartridge type of air purifying respirator is listed in the MSDS as acceptable breathing protection. The general health effect of isocyanates is something like asthma. It can occur shortly after use and the severity is different between individuals. There have even been some deaths from using it. In my opinion it is not something to mess with. My own experience with using it w/o a supplied air respirator was a rather severe shortness of breath and a hacking cough that started a few hours after exposure and lasted for a couple of days. Here's a good, short, write-up about isocyanates. Also here's what Len Stuart, the pro painter that is the proprietor of Autobodystore.com has to say about isocyanates. There is no "remedy" other than using the proper fresh air breathing equipment or staying away from products containing the isocyanates in the first place.  Rod
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