Hello cjensema!
Fixin' this depends largely on how the soldered joint is looking and what sort of faults you are facing. The following describes my methods when soft solder is used. Silver solder is another, much more difficult problem.
If it is large areas without solder, like tiny holes along the guard/blade joint, i would do it over again i.e. heating the guard, removing the blade, cleaning the joint and soldering again.
If it is only tiny uneven spots without solder, you may cover the whole joining area with flux and heat the solder again and eventually adding some solder.
Flux is very important. I use lots of it. I clean the joining area of blade and guard before soldering with great care, with acetone to remove any fat, wax or oil. Then i cover the whole areas with a small amount of flux before assembling the whole thing. After assembling i take a small, very thin piece of bamboo with a broken end which serves me as a small brush adn dip it in the flux and let two or three drops run into the joint. I cut small flat pieces of solder and lay them where they should run in and heat the whole area with a big, not to hot flame. I try to cover the whole joint with the flame to avoid oxydation. If necessary, i add small amounts of solder while heating. When i see that the joint is completely filled with solder i stop heating immediately and let cool slowly. I remove excess solder from steel with self made brass chisels, from brass with self made copper chisels. To clean up the surface you can use the carbon fiber brushes for dremel or proxxon tools, too.
Hope this helped a bit.
Achim
