to prevent rusting you need a 3 step approach - forget the coal tar - its useless.
the supplies you need are 1/2 to 3/4 kg wax (bees wax = best otherwise regular candle wax also can work) - some light mineral oil about 1/2 litre - any oldest school engine oil and some kerosene oil - about 1 litre, mix em up and leave alone for 4 days. (the wax should be about equal to the oil
a garden spray pump (the thing maali uses to spray your flowers with pesticide and a long plastic pipe with closed end - punch a few holes in the end to make a spray end.
a container of 3M undercoat.
remove the interior door cards from the doors and their plastic sheets too, then remove the bumpers and lights, fully wash the car at a high pressure facility and now remove the rubber plugs you see in the rear fender wells and push your plastic wand in there connected to the garden sprayer with oil and spray the mix in there - dont flood it you are only coating it. (install the rubber plugs back)
spray the mix in the door cavities too and also in the bonnet frame and boot lid cavity - some even up where the glass seal is. take wand and insert it into the cavities left by the removal of head lights and tail lights. This gives a good wax coating inside the body where you normally dont clean.
spray in the doors too.
lift car and tape off the areas you dont want coated - make sure the door sill (footboard) taping is done such that you get undercoat about 2 inches on the panel. Apply the coat to the metal parts only, if your old coating is soggy from the sarviss stashun aail isspray then you need to scrape that off first.