depends what your welding but heres what i do at work

1. grind off all paint on and around the affected area
2. cut out or grind rust away
3. weld up or weld plate
4. grind down welds if required
5. etch prime the area
6. filler or filler primer or primer depending on finish
7. sand back and prime ready for paint etc

this is the basics to every weld repair,
weld primer is only really needed if you intend to either weld outside (not recommended) or welding over a period of time