Hi Gary
Your collectors will be fine, the silicon, acting as a thin flexible bonding agent once set will still allow some differential expansion whilst keeping the fins stuck to the risers, filling the voids and effectively sealing out any moisture ingress; especially if copper risers are used.
I found copper fins easier to work with than AL. easier to bend, very fast to solder after clamping and using a gas torch and a bar of stick solder; but possibly requires more specialist tools. Also thin soft aluminum coil stock is not readily available in NZ, unless purchased in 250 Kg rolls, the alternative is a harder alloy and more difficult to work with.
Cheers
Mike
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