If your only after rustproofing then I wouldnt go the bitumen route, its going to simply add weight that you dont need (I`m sure the hinges will agree with me!) & if you go too mad it can block the vents, create loads of mess & smell & divert the water to other places that`ll rust instead....
On very hot days, expect it to turn into a semi liquid, dripping down the insides of the doors, blocking up the lock mechinism, dripping out the door drains, down the sills, on the floor/road/drive/garage floor, neatly allowing your to trample it all over your carpets in the car or house without realising...
The other pain in the arse think with bitumen is if you have to work inside the door in the future (window mechs, locks, speakers if fitted etc), you just end up with tar all over the place, on your clothes, ripping the hair out your arms etc...
"Zink" spray only works on bare metal. Ideally you dont want to rub down to any bare metal as that would be removing the galvanised layer.
Zink paint is bo***s in my opionion. People seem to think they spray it on (often over other paint) & like a miracle, the cars galvanised... if it was that easy, every car would be galvanised! its just glorified primer.
Rust doesnt occur in the middle of vertical panels, it occurs when theres a join, i.e. round the edges where it joins the frame, round the hinges & locks & round the strengthening beam where it sometimes has a slight gap between it & the skin, the gap is usually so small it slowly builds up dust & grit which attracts moisture & slowly rusts it out.
Also the seam sealer (between the skin & frame) & blobs of adhesive (attaching the skin to the beam if fitted) eventually harden & crack, creating unpainted crevices for water & dirt to build up against bare metal.
Buy yourself a Waxoyl aerosol & simply go round the areas mentioned above with the long thin nozzle provided.
Waxoyl works best when warm, it `flows` into crevices better when put in a pan of warm water (bath temp, not boiling or it`ll explode!) for a few minutes prior to using it.
Also remove the window seals to check it hasnt been rubbing through the paint & rusting, if it has, treat it, then cut sections of insulating tape & put them where it was rubbing, then rub over the whole lip with vaseline.
Take the opportunity to clean the dust & grit off the inside of the weatherstrip, that`ll stop your glass getting scratched. Then replace the weatherstrip.
Do the same behind the mirror gusset.
Somewhere else that rusts in that area is the door step part of the sill. This seems to occur when dirt builds up between the sill & weatherstrip.
To get rid of it, undo the screws on the carpet edge trims, remove them, pull the weatherstrips off the top of the sills, give them a thorough wash & brush, treat any rust behind them, paint, rustproof & put it all back together.
If you`ve got time, take the rear trims off the inside of the rear wings & do the same thing round the outer to inner arch joins, then jack the rear up as high as your axle stands can safely go (so it flows down the lower inner to outer sill joins to the front end & doesnt just drip out the first hole). , tape up the sill drain holes & spray (with the wide nozzle) a load inside the lower rear wing & sills Remember to remove the tape after.
Ideally you should do that with a proper sill rustproofing kit which is basically a pump with a 4` length of washer tube with the end hole blocked up a loads of small holes round the end of the tube, you shove the tube all the way down your sills from the other end, pump up the can with the pump & press the trigger, slowly drawing the tube back through the sill, this completely coats the inside of the sills & inner membrain panel.
If you`ve got any more time, unbolt the lower wing to sill bolts, clean all the mud & dirt out & spray it over the inside of the lower wing & upper sill join.
Remove the headlights & indicators, look up under the slam panel, theres likely to be rust there (especially where the front panel joins the inner wings behind the indicators). treat it, paint it & rustproof it.
You can do the bottom of the boot lid & leading edge of the bonnet too.
Externally, the main problem areas are the rear turret to inner arch joins & the seatbelt reinforcement points. its best to use a stonechip type paint here (after dealing with any rust).
The underside will have a rubberised underseal on from Fiat. This is great when new, but age hardens & the leading edges of the sills & floorpans get a constant stonechipping from crap thrown up by the wheels, this breaks the rubber underseal & causes water, in effect, to be jetwashed between it & the metalwork every time your drive in the wet (& 50 times worse when theres salt on the roads or the cars regularly near the sea).
The underseal then starts to do more damage than it prevents, causing LOADS of hidden rust to occur in a giant rubber water sack! The process starts to occur after 6-10 years after the cars made depending on mileage, how well its been maintained/cleaned & if its been kept on top of (Fiat were suposed to reapply the rubber coating every year during the rustproofing check for the anti corrosion warranty, you know, the one no one ever bothers with!). The First you`ll know about it is an MOT failure for rust, & when you start to pull back the now loose & damaged rubber underseal & find a rust patch 6" bigger than the 2" hole you could see, you`ll realise how it fails.
If your floor isnt rusty, I`d recomend to you extend the lower part of the factory under arch mud shield by 2" so the stonechipping doesnt occur in the first place.
Repeat the rust proofing every few years & your car (any car) should remain rust free indefinately!
Oh & I had a P6 as well, a 1975 2200 in burgandy. Not fast, but felt very nice to be in. I also had an 86 2600 Vanden Plas auto in moonraker blue which I loved.
P6`s rust rather well too dont they... (no, its not mine!)
