I tried this yesterday and it worked a treat. My heatgun also only had a 500'c setting and I used the tutorial
HEREJust keep that gun 3-5" away and moving all the time, I only did for 3 mins (smoke alarm in hall went off!!!) then turned over and done the back as well.
your may take a bit less or a bit more, trial and error, my advice is start slow.
Good luck