mark2 - sikaflex is a great product - but no chance of ever getting the skins off again. I used brush on sound deadener to isolate mine from the frame. A non-drying product like windscreen sealant would be fine though. I have heard that coillers use sikaflex/glue, and you have a 50/50 chance of getting the skin off or buggering it.
I didn't anneal my skins, and 14 years on - no complaints. I think they are pretty soft. I did "heat shrink" (anneal) some high spots when panel-beating them (heat with an oxy till it changes colour then hit the high spots with a wet sponge.
My skins are VERY tight. A "bit of heat" would probably anneal (soften) the ally - right at the join.
I have HEARD plenty of galvanising horror stories, but never experienced one. Minor distortion, bits of crap mixed with the zinc, small patches of missing zinc (touched up with cold gal) - but that is all. If you are a perfectionist, paint is probably better, but if you want it to last forever...

