With the doors dismantled I could see the state of the door innards with regard to rust. The bottom of the door skins are one of three main rust vulnerabilities on early 308s (the other two being the panel behind the front wheels and the panel behind the rear wheel). The problem with the doors is that water enters past the window glass and runs down inside the door.
There are two drain holes at the bottom of the doors but they are covered by the weatherstripping on the outside so water cannot drain away. Furthermore, the drains are not necessarily at the lowest point of the door so water can pool without ever reaching the drain holes as in my case.
Ferraris had no significant rust proofing until a basic process was introduced towards the end of 1983. Nowadays significant rust protection can be achieved using a product called POR-15. POR-15 is an anhydrous urethane paint that cures on exposure to moisture. It is extremely effective at encapsulating the rust forming a glassy, hard finish over the metal/rust that is chemically impermeable to water and oxygen… the necessary ingredients for rust.
POR-15 requires a systematic preparation procedure. If it’s not followed I have found the final results can be variable. However, if the instructions are followed to the letter, the results are remarkable. There are two downsides. The first is POR-15 degrades in UV light. So anything painted that is exposed to sunlight needs to be further protected with a UV-blocking top coat. Not an issue for the inside of my doors. Secondly the stuff is not nice to use. It is highly volatile producing solvent-heavy fumes. It cannot be thinned when wet or removed once dry so gloves and neat working are a must. Finally once the tin is opened it starts to cure even if after the tin is closed. Once curing has started and the tin closed you will never get it open again once the paint has gone completely off. For this reason I only small tins that will be consumed within a day or two of opening.
Step one is degreasing for which they provide a product called Marine Clean. This is an OK, but not a great, degreaser. Step two is to prepare the area with a product called Metal Prep which seems to contain both an etching primer (to prepare the good metal) and phosphoric acid (to convert the rust). The final step is to paint on the the POR-15. It has good leveling properties so even when brushed on, it leaves a smooth, glossy permanent barrier to rust. Enough to keep my doors rust free for a few more years.