Capillary action will work properly only when the surfaces of the metals are clean. If they are contaminated, IE: coated with oil, grease, rust, scale or just plain dirt, those contaminants have to be removed. If they remain, they will form a barrier between the base metal surfaces and the brazing materials.
An oily base metal, for example, will repel the flux, leaving bare spots that oxidize under heat and result in voids. Oil and grease will carbonize when heated, forming a film over which the filler metal will not flow, and brazing filler metal won't bond to a rusty surface.
Cleaning the metal parts is seldom a complicated job, but it has to be done in the right sequence. Oil and grease should be removed first, because an acid pickle solution aimed to remove rust and scale won't work on a greasy surface.
If you try to remove rust or scale by abrasive cleaning, before getting rid of the oil, you'll wind up scrubbing the oil, as well as fine abrasive powder, more deeply into the surface. In most cases you can do it very easily either by dipping the parts into a suitable degreasing solvent, vapor degreasing or by alkaline or aqueous cleaning. If the metal surfaces are coated with oxide or scale, you can remove those contaminants chemically or mechanically.
Other Variations of Cleaning Brazing Metals
For chemical removal, use an acid pickle treatment, making sure the chemicals are compatible with the base metals being cleaned and that no acid traces remain in crevices or blind holes.
Mechanical removal calls for abrasive cleaning. Particularly in repair brazing, where parts may be very dirty or heavily rusted, you can speed the cleaning process by using emery cloth, grinding wheel, file or grit blast, followed by a rinsing operation. Once the parts are thoroughly clean, it's a good idea to flux and braze as soon as possible. That way, there's the least chance for recontamination of surfaces by factory dust or body oils deposited through handling.