We buy a cheap hand pump sprayer. Fill it with POR 15. Put the nozzle down in the crevices and pull the trigger. Put paper down as it will drip out. Let it harden up. We've been doing them like this for over 15 years. Any area you really can't get to. Doors, cowls, etc. My tow vehicle is a rust free 1995 D3500. It has the original doors done this way, no rust. Living in Michigan it's a miracle.
Doug