Hello Martin  
 
When you have mixed the proseal you will have limited working time depending on temperature
Although you will notice that the longer you work with it the stodgier it becomes so t keep it warm,.to answer your first question no don’t let it set up before working with it.
Here are a few tips  
•      Before you start prepare everything you need such as latex gloves /  MEK or thinners / cleaning rags etc ( to be able to clean up)    
•      Rough up the areas with emery cloth or similar and clean down with mek or similar ( to remove any grease etc )
•      Make sure that you thoroughly mix the proseal  
•      Tape up the areas you don’t want pro seal to get on its easier to clean up  
•      Don’t skimp  with the proseal if you do you will end up with leaks  
•      You will be starting with the tank stiffeners use rivet tape to hold all the rivets in the skin use your back riveter to set these  
•      Apply to both surfaces and also apply in the rivet holes  
•      After riveting apply a small amount of proseal over the shop head of the rivet .
•      Don’t try to do it all at once take your time  
•      To be perfectly honest i have read that some builders don’t like this part of the project but I really enjoyed building the tanks  
Good luck  

Thanks guys