James and Tony:
As a weldor, I can tell you that: a) the reason your headers likely need repair at these joints is because they were not welded with good penetration during original construction(or were subject to "work-hardening" because of inadequate support) Since this is one of the thicker parts of the header, it is unlikely that corrosion alone is the problem, since other thinner parts of the header would have corroded away first. b) (because of the above) any welding repairs you make on the surface without removing the header are likely to be varying degrees of temporary - until the next leak occurs.
I would advise removing the header, and if it really is solid, doing an inspection of all four pipe connection welds from the inside where you can usually see how much corrosion (and little penetration) has taken place. Then, if repair is justified, you may be able to catch all potential leaks at once, grind out welds and ensure good penetration, and use a die grinder to smooth the inside if necessary.
If all this sounds like more work and expense than buying a new header, then be very careful in examining the welding on the new header, since this MIG welding is usually done by a robot, and often may not be closely inspected. Also, consider going for the extra expense of stainless steel.
Of course, if all you want to do is get back on the road...?
POR 15 makes an exhaust pipe coating - others may know if it works well.
Hope this helps!
Ron Tebo - Series I