Don Y wrote:
Hi,

We have a "double wide" (17'?) *insulated* garage door. As such, the
internal structure of the door is largely hidden.

Across the length of each of the "panels", a metal "stiffener" is
attached. Essentially, like a metal 1x3 fastened "edge on" for
greatest rigidity.

All of the fasteners to the door itself are large, but short, "sheet
metal screws". There seem to be certain places in the door where
"internal members" are present -- beyond just the sheet metal "skin".
So, only screws installed in these locations can actually carry any
load.
These seem to work their way loose over time. Making the INEVITABLE
mistake of trying to lift (or lower) the door by grabbing onto one of
these stiffeners invariably ends up loosening screws that attach it
to the door panel.

As they are sheet metal screws *and* the interior of the door is
inaccessible, there's no real way to tighten them; you end up
having to move to the next larger screw size.

[There's a limit as to how far you can go with this!]

Short of putting carriage bolts *through* the door (the exterior is
"smooth"), what other approaches can I take to secure the stiffeners,
hinges and other hardware to the door panels?

Threaded pop rivets (to give threads on the hidden side) and bolts.