I need to airbrush a 4 tier cake frosted with IMBC.  

The good news is I am only applying an allover pearl sheen.  

The bad news is there will be high humidity.  This means my buttercream starts to SWEAT almost immediately.  I'm not sure how to proceed, what to expect and how to troubleshoot.

Assuming I get the paint on quickly, will the color start to bead up once the cake starts to sweat?  

This is an "in house" event, so I will be assembling and airbrushing on site and walking the cake next door where it will invariable be outside under a tent or inside with doors open.  At this time, the forecast is for only 20% rain with 75-80% humidity.

Thanks for any advice!