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Hi Jason, 1. The need for a fresh air respirator system comes about with the use of isocyanate-containing additives. Typical are the hardeners used in enamels and the hardeners (also sometimes called activators) used in urethanes. If you don't use a hardener, then the use of a decent fitting cartridge mask is acceptable. A fresh air system is a special respirator that is fed fresh air under pressure directly to the mask. There is no filtration of paint products involved as there is with a cartridge mask. The fresh air equipment is relatively expensive, starting at around $400 and going up from there. 2. The epoxy primers from PPG (and probably similar products from other manufacturers) do not contain isocyanates in the additive (catalyst) so those products do not require a fresh air system. This is the exception rather than the rule. Most other two part products contain isocyanates in the second additive part. When in doubt, always check out the manufacturer's safety data sheet for the individual additive (MSDS). These are usually available on-line from the manufacturer's web site. 3. If you go past the time window of three days for the MP-170, you really should scuff it and reapply a coat. That's pretty easy to do. Use a general purpose 3M hand pad (Scotch-Brite) for the scuffing. This is to get the best possible adhesion, not only with the underlying older epoxy but also with the topcoat that you will be applying within the new 3 day window. 4. Forget lacquer thinner for final cleaning. It is not the appropriate product for that. Get some PPG DX330, Acryli-Clean. See also this link.  Rod
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