Regarding your use of the heat gun, mine has a high and low speed setting. How far do you have it from the work and for how long? I would err on the side of caution myself.
Yes it takes a bit of trial and error! Depending on how powerful your heat gun is, I'd use high speed and keep it a couple of feet away and keep it touch warm. Careful not to burn yourself though!
We usually used the hot air gun straight after painting the part to speed it up to the 'touch dry' stage. The main reason was to get it dry enough to bolt it up to the tractor rather having to wait a day for it to dry as the shop rate was $110 per hour and management based efficiency on the turn over of machinery per day.
The rotten thing is the mechanic who just ticks off the pre-delivery sheet on a new tractor without actually checking it gets promoted and the other mechanics who do the pre-delivery properly get hit with penalties and even threats of the sack if they don't get more vehicles through.
When the customer gets charged for a 'service' don't expect the filters or valve clearances to be adjusted!
Maybe you could try some paint on a similar metal and experiment? Once the paint bubbles it has to be redone.
The other thing we did to prevent the 'orange peal' effect when painting in cold weather, was to put the can in a bucket of hot water.
