From reading about issues with die cast model cars, they call this "paint rash", which is not the same as "zinc pest" caused by bad alloy metal and which results in failure of die cast parts.
Paint rash appears to be caused by having too much air trapped in the die cast metal at the time of casting, and is affected by casting too quickly, or inadequate die design to allow for air to be pushed out, perhaps other things as well. From what I can tell, the good news is that paint rash does not necessarily indicate that there is any problem with the metal, but is only a real nuisance as far as finish is concerned. I have seen this on many of the items that were made by Samhongsa for Lionel (by MTH at the time), including the 18010 turbines, the 18009 Mohawks, 18018 Southern Mikados, and especially the 51501 semi-scale hoppers. As Harmonyards wrote, the blistering paint happens but it doesn't seem to affect the model structurally. Time will tell, of course.