[Caddylackn]

Pin holes are just the tip of the iceberg. The real damage and pitting is on the back side that you can't see. All around the pinholes, the metal will be too thin to either grind or weld to. Any other repair method other than cutting out the rusty metal and welding in a patch panel will only be a temporary band aid, and the rust will come back.

You can try a rust conversion product that converts the properly prepared rust to hard surface but that is not a fix for the pinhole. It looks like salt road spray was getting forced back there. That area behind the panel should be sprayed out to remove the salt coating, then loose rust brushed away, cleaned again, then treated with a rust conversion product to protect the existing areas from further rust out.

The pinholes repair is the tough part. You will find you need to remove at least 2" from all around the pinholes to get to existing metal strong enough to weld in the repair panel. Once that rust area is cut out, then you have access to the rest of that area behind the panel, to clean, prep, and apply rust converter.

I would consider apply a undercoating material over the back of the panel once the metal repair work is done and all the rust converter on the rest of the panel is cured. This will help future damage from salt and road wash.