Crud in the vent, how do I get it out?

Depending on your car you can cut a good sized hole from the engine compartment side or cut a large hole from the door hinge side. That will give you great access so you can clean it out properly then undercoat everything. Then you just weld up the hole and paint. But some folks don't like cutting up their car. I would much prefer to get the area good and clean and undercoat because IT WILL RUST in there!!!

OH, I forgot......the cowl drains (including all the crud) into a pocket at the back (bottom) of the front fender. So if you don't also clean out that pocket the rear/bottom end of the front fender will also soon rust!!!!