I used WD-40 on my tent zipper, and it's been smooth as butter ever since. I used the standard one which could possibly damage the fabric, but it's good for me, no damage. There is also a special WD-40 Silicone Something, which is supposed to be better but i didn't have it.
I tried with candle wax but it only made the zippers dirty and not smooth.
I tried with candle wax but it only made the zippers dirty and not smooth.