What Kit said, although I use lacquer thinner to do the initial cleaning. Acetone can have oil, or other contaminants in it.
You have to clean the whole area leading up to where the guard goes, the whole handle, and choil, as well as the guard itself.
I then take it into the kitchen and clean with dish detergent, and the hottest water I have, and then use something like 409 cleaner.
You have to do this until the water sheets across the blade when you rinse.
If it's not clean enough, it will bead up, or run in rivulets. Just keep cleaning till it sheets across the work. You will know it when that happens.
95% of solder problems can be attributed to poor cleaning.
After it's clean, don't touch the areas to be soldered, or the tang at all.
If you have to slide the guard up the tang, you don't want to contaminate the inside of it with fingerprint oil, etc.
I use a cotton swab to put a coat of flux inside the guard, and then around the blade where the guard will go, before sliding the guard on. You can put a bit more on then, if you wish.
If the solder turns black while heating, you've burned it, and have to stop, clean it all off and start over.
I heat from the tang, and the bottom, and sides of the guard, moving all the time. You can have a small piece of solder laying along the juncture to see when it gets hot enough, and when it flows, run the solder around the joint, followed by a pick of sorts that has been heated, and has flux on it(I use an old pointed swiss needle file). I just draw that around the joint after. It helps to keep the solder from pulling back, or forming bubbles.
You can get excess off with an acid brush dipped in a bit of flux, and just sweeping, or brushing the excess, still liquid solder off the guard, or you can use steel wool wrapped around a matchstick to wick it off.
All this takes place very quickly, BTW, so have everything ready beforehand.
Let the joint air cool. Don't run it under water to fast cool, that will lead to a weak solder joint.
Before soldering, I also wrap a damp washcloth around the blade, above the solder line, and hold it on with mechanics wire. Helps keep everything from overheating, and possibly ruining the edge temper.
Of course, the above is with the understanding that you're trying to solder "normal" guard material. If you're talking about 300 series SS, all bets are off. That stuff is almost impossible to solder.
:barf:
Edited to add; I forgot to say, when you get to the point of the solder melting, and you run the solder wire around the top of the joint, don't forget to run it around the bottom.
I always start on top, as I never had any luck with the solder wicking up from below that some claim to do. I just run the solder top, and bottom.
BTW, you have to keep the torch going, and handy, which can provide some interesting moments if your coordination is a bit off. I believe the yuppies call it multi tasking!!
