I use POR-15 on the (interior) floor, firewall, under the cowl, in the cowl ( old fords) and in between interior and exterior panels. I use a cleaning wand hooked to the shop compressor to spray it in the tight areas like between panels and where sheet metal over laps and has a gap. Drill a holes in the rocker so I can shoot POR-15 in from differnt angles. fill holes with evercoat product. POR15 sells a filler too. Never tried it tho.
No bondo. Evercoat products or better.
There are some good industrial coatings from sherwin williams. I have several gallons. I have used it on the metal roof on my house. (I'm high class trailer trash). It has stool the test of time in that application. Have yet to test it on a parts car. I wat to try that on the bottom side of the floor pans, then under coat.

I noticed in the Mercedes service manuals that leak prevention is stronly stressed. Inspecting all window seals very carefully. These cars are very well sealed on the floor and firewall and water does get in, it is trapped. Making sure water is not getting past any seals is going to be my #1 defense once restored.

I have tested rust converters and they all failed horribly. And POR-15 works only if the surface is preped correctly. I am wicked happy with POR15 after 5 years of using it.
My rule of thumb is, "rust through" rust must be cut out and surface rust can be sanded, blasted, ground off and treated if the metal is not left too thin.