Depends on the surface. most of these companies suggest rust converters such as corroseal on 100% surface. not mix rust. in that case it works very well.

while POR15 will work, I think its just impractical. you will never get every surface, and you wont get inside the frame of our trucks.

I'm opting for heavy degreasing of every surface. Using salt away. applying corroseal on the very bad areas and fluid filming the rest.

I think POR15 would be good if you take apart the car. but its such a pain and if you dont get 100% you have to go keep touching up.

At least with fluid film you see whats going on, and not covering it up. rust never sleeps. so you should see whats going on and keep adding till you get the best coverage possible.

I'm on the fence if i want to spray paint before fluid filming. This will act as an indiciator too see rust is growing under the fluid film. Not sure if this would cause pockets for rust to grow. For now winter is here, I'm fluid filming whatever I can then going through a more indept process in the summer.

Thanks