I think you are right that it was spotwelded and then sealed - not sure if it is epoxy
but sealed anyway. Drilling spotwelds is a bit tricky for several reasons one is that
the rapid cooling from spotwelding can harden the soft steel in spite of its low carbon
content. (I tried spotwelding some AISI 1002 steel when I worked at Kelly AFB and was able
to get it up to Rockwell C 37, which amounts to an equivalent of 180,000 PSI tensile strength.
What this means is that if your drill has a chance to wander off the weld, it will because
the adjacent body metal will be much softer than the spotweld nugget. Therefore it
is pretty important to start with a small hole that will be fully contained in the nugget.
To do this, if you can't see the spotweld clearly, a little sanding over it will show the
indentation still painted when the paint is sanded off around it. I'd start with a small
drill, 1/8" or smaller and after drilling through and then drill bigger till the nugget is gone.
--
George Downs, Bartlesville, Heart of the USA!