The key to a successful remedy is to totally remove the oxidized surface. Otherwise, the rust will keep on truckin' even if you cover it up.
How you accomplish that depends on how bad the lap weld degeneration is.
In our case, I caught it and dealt with it before it advanced too far.
I will say that, regardless of the degree of rust, a multi-coat approach will likely still be needed, with the following sequence having been done in our case:
4. Latex primer
5. Latex encapsulation, multiple coats
That ^^ remedy has held for over a year now, with no visible signs of further deterioration.
If you have worse rust than what I show in this blog post here
, then you might be able to do a good body filler in conjunction with those other steps.
- BUT -
any body filling you do will still need to occur AFTER you remove the active oxidization surface. It's very important.
And if the rust has progressed so far as to involve the vehicle's ribs, well, I don't think you will have much choice but to do structural work. Brace yourself - it's fabulously expensive. If that is the case, the cost could be a large percentage of your Interstate's market value.
Pics would help this discussion substantially.