Extensive testing over the years in many places has shown that the best surface (generally speaking) for brazing is the "as-received" (as-rolled, as-drawn, as-machined, etc.) surface roughness of the material coming into the shop. This as-received surface may be 16 RMS, 32 RMS, 64 RMS, 125 RMS or even greater.

Surface roughness adds surface area to the joint, which provides countless extra capillary paths for brazing filler metal (BFM) to follow.

Additionally, and very importantly, it allows parts to be assembled without adding any "spacers" in between the surfaces to be brazed. But it allows so-called metal-to-metal contact between the parts being brazed. The surface roughness of the faying surfaces should be quite sufficient to allow BFM to flow through the gap between the surfaces that is created by the surface roughness of each component.

It is NOT RECOMMENDED to polish the metal surface prior to brazing. This can destroy capillary paths and, with metal-to-metal contact, could actually shut off capillary flow entirely.