Hi R_E_Ren,
The types of material being welded play a large part in choosing the proper welding rod. Not knowing whether the metal is a mild, low alloy, or nickel steel, or if it has other melted components like chromium, manganese, or vanadium in it, and knowing that each uses different rods to ensure proper metal bonding, it may come down to just settling for a mild steel type of rod and hoping for the best. Once the rod is determined, as previously mentioned, the surfaces to be welded must be clean of rust, dirt, grease and grime. A wire brush will go a long way towards cleaning the area to be worked on. If it is a stress crack which is being welded, take a hand grinder and grind a 'V' the length of the crack in order for the weld to penetrate both sides of the break, as mentioned by HTH. revpol seems to have some good advice, although I don't understand what problems could arise from grinding a v groove in the crack for better penetration, if it is properly welded. After each pass chip off the slag and debris and wire brush the area before another weld is made. Cracked areas hiding under rusty metal flakes and/or layered metallic garbage should have the debris ground off so the weld will be made against the parent metal or the most basic of structure. If there are multi levels of paint on the piece then take a grinder and work out the paint so the arc will strike easier and so the weld will have better integrity. When the final weld has been cooled and the slag and debris cleaned off of it, inspect it carefully to make sure it did what you wanted it to. Look for undercutting of the parent metal as this can form a weak spot in the repair. Check to make sure there is ample bead on the repair and that any patch plates you mentioned using have good penetration on their edges. A good coat of primer and paint on the repair will protect the bare metal from the elements. Welded repairs seem to rust quicker than any other type of exposed metallic areas. Besides that, a good coat of paint will look good and make the repair look complete.