Why your job was unsatisfactory:

#1: Use of inappropriate tools. A torch is the wrong tool for the job.
You wouldn't have had the burn-through problems with a MIG welder. A
MIG welder puts out much less heat so you probably wouldn't have
caught your undercoating on fire.  You wouldn't have had the rust
problems with a sand-blaster. You wouldn't have had the problem
getting the old part out if you'd had aviation snips and a dremmel
tool (a very SMALL grinder/cut-off tool).

By the way, on my bus the battery tray is spot-welded to the fender.
To remove the battery tray, drilling out the spot welds will do quite
well.  Then if you're using a MIG welder, you can do "plug welds" to
the new part through the holes. Of course, this also will require that
you re-paint the fender, since you're going to have to grind your new
plug welds flush with the fender. For more strength do a full-scale welding
bead. Again, the right tool (a MIG unit) makes the job much easier. 

#2: Poor surface preparation of new part. The welding bead area should have
had all contaminants such as primer and galvanic coating sanded off prior to 
welding. 

#3: Poor fitting of the new part. Mangling it to get it in there was *NOT* the
right thing to do. The new part should fit like a glove. 

A very professional job could have been done with the right equipment
and an experienced weldor (person who does welding for a living, as
vs. the guy down the street who has an oxy-acetalene torch). You chose
not to have it done the right way. So be it. I agree, if you're not
going to do it right, just treat the thing with rust remover and then
fiberglass it. But that doesn't mean that welding new panels in is
necessarily a disaster. You just have to know what you're doing.

--
Eric Lee Green    elg@usl.edu     (318) 984-1820
        P.O. Box 92191  Lafayette, LA 70509