Soldering the way that seems to work for me
Reading the forums over the past year I noticed several people were struggling with silver soldering. Though I've had previous soldering experience, I counted myself among the frustrated. This method just kind of evolved for me - out of necessity. It developed out of a combination of experimenting and tips from the Bladeforums. What I like about it is that it is fast, predicatable and does not mess up that blade you have spent so much time getting cleaned up.
I use a small C-clamp to hold the guard in place and then clamp the tang into a vise. All the surfaces are clean.
This is the crucial step for me. I take two lengths of silver solder, bend into an elongated "L" and place them where the tang meets the blade. Next, I apply some drops of flux around the whole area using a small piece of wood as a dropper. Its now ready to go.
I use a propane torch. ( I've also use a welding tip on an Oxy - Acet. torch.). I hold the torch tip from underneath at about a 45 degree angle - 2-3" away. I aim so that half the flame hits the tang and half hits the guard. I move from side to side. When the flux starts to boil, I know the solder is about to melt. As soon as its starts, I linger a second or two , shut the flame down and use a small brass rod to help the solder along. About half the time, this last step is not necessary - the solder will flow right into place a few seconds after it starts to melt.
The photos above show how things look after a quick pass on the buffer. My first attempts at making a clean joint were frustrating because of all the clean-up I had to do with various grits of sandpaper on the guard and the ricasso. I make much greater use of the buffer - but be careful!! a buffer is a vary dangerous device! I first wipe the blade down and then buff using a green compound on a sewn buff. Its now ready to finish out with some brass gravers.

I make my gravers out of solid brass rod, both square and round. I grind only a slight angle on the end. The square rod I round on one side. This work great at creating the initial round groove between the blade and guard. Push this along the groove and the round side shapes while the flat sides remove the excess material on the flats. I usually buff this area again after shaping, come back and finesse it with a round rod graver, clean up any flat areas and buff again.

This is the area after the second buff. Reflections obscure the finish some, but it is just about done. The part I like about this process is that I did not use one piece of sandpaper. I started out with a mirror finish blade and ended with the same finish. The guard still has a few scratches to take out, but that is minor compared to what I used to do. I've used method this with satin (sanded) finishes also. It requires the re-sanding of the blade after buffing, but I have found that a sanded finish always benefits from a few more passes.
I put this up after seeing a suggestion from Gib Guignard to use small cutting/polishing attachments available from Rio Grande. This an assortment of small polishing wheels I just purchased from MSC and have to try. The idea is to avoid using a full size buffer. These and the ones from Rio Grande will chuck into a Foredom tool or Dremel. The ones on the left are medium and hard felt and the ones on the right are muslim buffs.
This photo is an enlargement of the gravers I make from brass rod. The one on the left is made from 1/8"" brass rod. The one on the right from 1/16" brass rod. As shown, they are a little dull.
Other Tips & Techniques from Forumites:

From Reg Ellery: Use the strips of solder, but try shorter lengths (about 1/3 as long). The heat can make it run along the crack.

From Will Courtney: " (from Bruce Evans)...just after soldering while everythings still good and hot, give the solder joint a quik spray with Windex. Not too much, just a shot or two, you don't realy want to cool the joint, the steam from the Windex blows the flux and other crud off the joint and helps neutralize the acid of the flux." also this one..."boil the blade and guard in a pan of water and baking soda for 15-20 minutes to neutralize any trapped flux. This keeps corrosion from showing up a week or month from the time you finish the knife."

BobVan Gelder / South River Knifeworks 476 Bardwells Ferry Rd. Conway, MA 01341
Phone: (413) 369-4482
Fax: (413) 369-4966