Whatever Wednesday: Why is my spray paint bubbling up!?!

I recently stripped and repainted an awesome bit of vintage kitchen plastic wrap holder a couple of weeks ago.  My friend had painted it with my favorite spray paint but called when it turned out that it had bubbled up over a large portion of the holder.

Then yesterday, my SIL texted me a picture with a project that had the same thing happening.

In both cases, they were working with metal objects.  And in both cases it could only be fixed by sanding the portion down with 220 grit sand paper, or by stripping the entire things, as I did my friend's plastic wrap holder.

So, here are some reasons that your spray paint may bubble up, especially when painting metal.

1. It is too wet outside.  This requires a longer dry time on first coats.  So, if you start putting on a second coat before the first one has time to dry, it will bubble.  Even if you're spraying in your garage, the humidity in there is similar to what it is outside.  We live in a pretty humid climate, and yesterday in particular was very foggy.  All those clouds hanging low did not allow for the paint to cure as quickly as it might say on the spray paint can.  
2. It's too cold outside.  Especially with metal, it is important to make sure that the outside temperature is between 50 degrees and 90 degrees Fahrenheit.  But remember that metal will stay colder longer than, say, wicker or wood, so it may need to be warmer or in warmer sunlight.  
3. It's too hot outside.  I haven't ever really had a problem with my paint bubbling up if it's too hot, but I hear that it can happen.  If I could guess, though, I would think that, at least where I live, when it's hot it's also more humid, so back to number 1.  
4. You cleaned your piece but didn't give it enough time to dry.  The object needs to be clean and very dry in order to make a nice smooth finish.  

If you do get the bubbling, sand with 220 grit sandpaper, wipe with a dry cloth, and then apply the spray paint again.  OR you can strip the entire thing down, but this is very, VERY time-consuming and a lot of work.  I mean, I did it because it was for a friend, and a client, but if the project were mine, I would have just sanded it and repainted it.

Here is the suggested dry time for my favorite Rustoleum 2X Ultra Cover Paint+Primer:
       
Dry and Recoat times are based on 70 degrees Fahrenheit - 50% relative humidity

• Allow more time at cooler temperatures. 
• Dries to the touch in 20 minutes, to handle in 1 hour and fully dry in 24 hours.  
• Apply a second coat or Clear coat within 1 hour or after 24 hours. 

  I always thought that spray paint dried and was ready for another coat after just 10 minutes, and I will say that in the summer, sometimes that's all it takes.  But always, especially in humid climates, it takes 24 hours to fully dry and sometimes a few days before I want to place other spray painted objects on it.  It needs to cure fully.