mick
there's some good info on that thread but also some nonsense. I worked in the metals industry all my life and whilst I am neither a metallurgist nor a paint scientist, I did pick up some tech stuff on the way. This is my non tech understanding of the tech problem
I cannot say whether BMW are using the best paint available and tbh I have my doubts if only from the car side. My last BMW car ( and the current merc) are both parked at the seaside when I go away on my boat and if a seagull aka flying rat, craps on them and the crap stays there then it burns through the top paint coat. Never did so on my volvo but BMW's best response it so say it should be washed off immediately. Great advice when you are away at sea. So as I say I doubt BMW are doing as well as they could and their arrogant Germanic "we are always correct" attitude stinks.
That said painting ally is difficult. Aluminium is very reactive ( clean ally without any oxide coat will fizz if dropped into clean water liberating hydrogen) and in air it reacts to form a thin oxide coat which dulls the surface but reduces reactivity. But add more dirty water as from the roads and more oxides and other ally salts are formed which always take up more volume than the metal they were formed from. The problem then it that at the interface between bare ally and painted ally, the more bulky corrosion tends to peel back the edge of the paint which reveals some bare metal and which then oxidises peeling off more paint and so on. Its as if the corrosion is burrowing under the paint but worse because the concentration at the paint edge leads to pitting..
The difficulty is that with engine casings and the like there simply have to be threaded holes, facing surfaces etc that are bare metal. So you have lots of these bare metal / painted metal edges. And you get even more as road pebbles etc chip through the paint down to bare metal. This is one reason why older bikes had bare ally cases - OK they go dull as the whole of the surface oxidises but they dont corrode an pit in the same way. The other alternative is to anodise the metal as for example with a sailing boat mast or those pink , blue, gold etc ally farkles you get on sports bikes. The gold stanchions on the GS are anodised. All anodising is is a thick stable oxide coating with a pigment to give colour
So what can you do to minimise the problem on GS engine cases?
1/ touch up any chips the moment you spot them
2/ remove water traps like the foam
3/ when you screw a bolt into an alloy thread use a joining compound like tefgel or duralac to clog up the threads.
4/ on machined bare ally that is hidden dont paint it but instead use something like waxoyl which cannot chip and provide an edge for corrosion
5/ if you decide to use stainless fasteners you must use duralac or tefgel on the thread. The oxide if formed between an alloy casing and a stainless thread will grip the stainless mercilessly and can make it impossible to remove the stainless bolt.
6/ avoid plastic coating. the coating may well be more resistant to chips but if you do get them then the corrosion seems able to travel long distances under the coating and to be far worse in my experience
I hope this is some help to you. I'm sure BMW could do a better job but the problem of painted ally is common to all bike builders. Incidentally, the comparison with cars is not valid since the same issue doesnt apply to steel. And the few cars with ally panels dont have machined thread in them for the corrosion to start.
