not all solders are created equal..... that and the flux used also can meant the difference between a clean seam, or just a hot clumpy mess.
ive had no issues with most of the carbon steels, but I was suggested to try this stuff, and it works great on Damascus, 1084, W2, O1, brass, nickel, 416 stainless, and have seen it used on the stainless blade steels as well. (will be trying it out with some S30V blades myself later this year)
I still use a MAPP gas torch, but hot air guns will get the job done a bit cleaner, and with less risk of messing up your blade temper. it also takes a bit longer than using a torch. I have a torch and not a hot air gun, so I still use that. just need pay attention where you put the heat, and not try to heat too much too fast.
http://www.knifemaking.com/product-p/jhsb.htm
hope this helps