Well, I mean is your metal stock getting rusty and you want to weld? Of course if it is aluminum, it won’t rust, and while certain classes of stainless steel will show a few rust spots here or there, rust typically isn’t a problem. What we’re talking about is good old fashioned mild steel. You know, the stuff that litters most welding shops and home garages of hobbyist welders.
It’s inevitable that surface rust will eventually form on mild steel if it is stored for long lengths of time in humid environments. One of the concerns that most welders face is the lost time spent in cleaning up old steel and trying to use it for their jobs and projects. Rust, like mill scale can create problems for porosity and inclusions if not properly removed before welding. Rust can be controlled if it is in long term storage via applications of lubricants, and pickling done at the mill. Even mill scale itself is a rust retardant, but eventually rust will get to anything stored long enough. If you are facing this issue, keep I mind the application you will be using it for. Not all applications require perfect metal. If you have accumulated rusty metal save it for these projects.
Also consider using filler metal high in deoxidizers like 70S-6 for MIG and TIG, and electrodes like E6010/6011 for stick. Of course, wire wheels work miracles to recover rusted metal stock. However, angle grinders used to remove rust at the point of the weld work better. Any remaining rust can be dealt with using an application of phosphoric acid.