Getting started with a hot air station.

[Yamin]

Hi guys, recently I got myself a hot air station to get started on SMD rework. I have been referring to a lot of online resources and have been practicing on scrap boards. However I noticed that removing components from certain boards are particularly hard. I'm assuming that on those boards the components were wave soldered on (not so sure though).
The setting I have set on my station is ~350 degree Celsius and the fan at around half. Could you please suggest or give tips on where to set the parameters at.
With these 'difficult' boards I tired increasing the temperature to 420 degree Celsius and then it seems to work, but not all the time. Would it be ok to go this high, generally speaking? How can you determine or know what temperature to use?
What effect does changing the air flow speed have? In other words what the significance of this parameter?
I noticed that under some components there were some flux like residue when removed from the board. What could it be? Could it be simply flux left over or glue? I would have included a picture but its not clearly visible.
Lastly the nozzle size, can using a wrong size have an effect on the result on the work?
Anymore tips/advice/suggestions on this would be greatly appreciated.

[Mr. Scram]

Parts on wave soldered boards are glued on before soldering to keep them in place. That's probably why you're having trouble removing them.