Oh yeah that's a constant problem...
It has been raining a lot here lately. I decided on a hunch to check the boot area because I read something online about them leaking. Well the boot lid doesn't leak it seems, but it does leak around the rear tail light from what i can tell, maybe a little from the other tail light also. And there was water on the bottom of the boot, particularly under the spare tire.
I ripped everything out and found there was water under this rubber mat stuff which I think is sound insulation and I had to start removing it with a chisel. It released in placed in large chunks where whatever glue held it in place has failed. I think it will all have to be removed so I can get it all dried out and the rust removed. So far the rust looks superficial, which is fortunate.
I'm considering how to treat the rusted areas.
-Remove the rust (rotary steel brushes, acid, perhaps sandblasting) and paint the bare metal with epoxy?
-Or zinc primer?
-Or nothing? Just use raw linseed oil and keep it covered and refreshed?
I am thinking I will both epoxy and then use linseed oil, particularly on all the seams to get the linseed oil into there and stop rust that I can't see. I plan to go over all the seams of the car with raw linseed oil. It creeps like nothing else and is very effective at stopping rust already inside seams you cannot see and then it oxidizes and creates a protective layer. Raw linseed oil is very useful at least on older cars.
What to replace the rubberized layer with later? Unknown... I think whatever I put in there should be easily removable again in case this happens again. I think this was meant to seal so water couldn't get in there but it clearly failed.
I need to remove the taillight to see why it leaks past it but very likely the seal is bad, I found some hardened sealer like sikaflex or silicone in there, crumbling away. Think it's some earlier attempt to fix it that now failed.
Also found these "flaps" on the bottom, they seem to open into the bumper if you push on them, they can open outwards but not inwards. Not sure what their purpose is, I would have thought they where venting the air, but that's what the grilles on the C pillars are for, unless these are just extras for the boot specifically.
