What I do to a bare shell that has been cleaned is try to coat everything that you cannot get to with a brush or a spray gun. The cheapest product is Black rust-o-lium . I tape all the weep holes and plug hole in the rear quarters one at a time. Fill the quarter with rust-o-lium paint.
After it is full and you splashed some with a brush up into the outer wheel house. Open the front weep hole or plug and drain it back into the container. then move to the other side.
Through the rear door panel hole in the interior fill the rocker area to get it between the quarter and rocker after taping the weep holes shut. If you have enough paint pour it full and drain it back into the container. You want to coat where the water travels to the weep holes to slow/stop the rust. I have a perfect body here That I intend to coat the rails inside with a wand and a slutz gun. This is still done on new Mack turnpike trucks after the plow and beds are installed. It is a mandatory procedure when built and required by contract. My son builds these trucks as a day job.
Here are picture of the rust free brown Duster ex-drag car I am putting back together. It was rust free I did not strip the under carriage but I did fill the quarters and coat the rusty surface with chassis saver. Due to it being very expensive I used paint to pour in the quarters and rockers. They sell 5 gal. containers of chassis paint which is cheap and the same as rust-o-lium . "Chassis Saver" is a different product, It is the newer POR15 product and is much better than POR15. I used them both.
Two days later and it was still dripping on the floor as seen in the last pictures.
There is nothing worse than rust bubbles coming through a fresh paint job where there was no rust to be seen at the time.
Start there.
I am also undercoating the car to make it look cleaner underneath. All new stainless lines were installed before undercoat but it is not necessary. the fasteners are all pushed through holes or bolted in place except for one on the passenger side which gets knocked in between the floor and the inner rocker face. I do not undercoat between the front rails.
Make sure you covered all the holes before pouring the paint or you need some family fingers to dam them up.
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