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I usually use hot air gun to solder components on the PCB. However I am afraid of using hot air on SMD LED - they have that clear plastic on top and i am afraid of melting it with hot air, so I usually used two solder irons to place SMD LEDs on the board, which is quite cumbersome.
I have regular solder paste and low-temperature paste (about 125 °C). Would it be safe to use hot air on these LEDs with regular or low-temp paste?
There's no way to be sure about potential damage without knowing the part number of your LED, which is needed to obtain its datasheet. The datasheet will outline the temperature specs of the LED (including max storage temp and max operating temp) as likely also its performance characteristics at various temperatures. I picked three random SMD LEDs from Digikey as a small sample (1 2 - the third is 160-1446-2-ND, but I lack the repu to post the link), and they have operating temps up to 80-85 degrees C. Max storage temp for one is 120C, but the other two max at 85C. Because of this variation, you need to know exactly what the specs of your LED are, but you'll might be cutting it a bit close. Some datasheets outline reflow info for their part, so your best method will be to follow that guide. Personally, I wouldn't be very worried about doing what you're doing as long as you have back up LEDs just in case, but if you don't have extras and/or can't risk damage to the PCB, then you'll do best to check the datasheet for all the info mentioned above.
I've had mixed results and its reasons like these that I finally built a reflow oven. My experience has been that if I use the hot air at 250 deg celsius more than about 30 seconds the clear lens starts melting. Usually you should be able to solder 0805 sized LEDs with a soldering iron and no need for a hot air gun.
