By Paul Nadel, Linus Pauling Jr., and David Trower
Paul Nadel: With the carpet out I drilled three 1 in. holes in the top of the three compartments along each longitudinal. I also drilled a 1/2 in. hole in the bottom of the front compartment for a drain which I later fitted with a removeable plug. I vacuumed out each section through the holes and flushed the whole longitudinal with a phosphate metal prep and allowed it to drain out the bottom hole. I then dried it thoroughly with compressed air and poured in a quart of thinned rust encapsulator into the rear section and allowed it to seep towards the front with the car slightly raised in the rear. With an extension on my air gun I blew the paint around each of the three compartments per side to completely coat the bottom and sides and allowed the excess to drain out the front into a can. When done I plugged the holes with plastic plugs and replaced the sound deadening and carpet over them so they don't show.
Linus Pauling Jr.: In my opinion, it would be a drastic error to put any corrosive agent (like rust removers and paint removers) into the longitudinals because it can never be gotten out. The grease-like cosmolene type rust preventives such as Waxoyl are much more sensible. I drilled 1/2" holes into my longitudinals (under the carpet) and sprayed Texaco Rust-Proofing Compound L in all enclosed spaces 25 years ago and have had no rust breakout since.
David Trower: Here is what I think is the best kept secret for preserving your 356.
Years ago on a nine-year old 356 I purchased, I drilled small holes under the threshold carpet and squirted engine oil from an oil can into the longitudinals. I also did the door bottoms, battery tray and various other vunerable seams and crevasses in places I thought moisture would collect and be held. The oil creeps everywhere, even up vertical surfaces. Keep all the drain holes open. Keep the oil away from brakes and rubber parts. Repeat the oiling process as needed as the oil creeps so much it dissipates.
The oil film is very thin and gets progressively thinner as it continues to seep in all directions over time. I do not advocate filling the longitudinal cavity with oil (or anything else), just enough squirts from an oil can to start the process of the oil creeping. It gets into everything, where it is needed.
I still have those longitudinals today (and the rest of the car). It has mostly original paint after 36 years ownerwship and many damp, wet, rainy, snowy, salty and downright dirty days of driving.