Had my 2004 dodge ram 1500 in my driveway sanding out the bubbles on the rust areas.
I removed both my rear tail lights and look inside the wells from there.

What is the best way to try and get some undercoating in there I tried a can but would not reach in there.

I would also try fluid film or some other oil to coat the inside wells with an oil coat. Problem is what could be used to get in there about 4 feet in.
I tried a garden chemical pressure tank with a wand but it sprays a mist I need something that will spray a stream to reach the 4 feet in.

Any suggestions