Thank you for everyone's contribution. Interesting thread as I have been mulling over fan sealant systems for my Maxxair 7500k installation. Based on obsessive research, at a minimum I would suggest a two pronged approach:
1) Apply a quality butyl tape between van and fan flange - this is your last line of defense should there be a breach in the primary sealant. Butyl also allows for easy future removal should it be needed in 10-20+yrs.
2) Apply the primary sealant (lap sealant; Sikaflex 521UV for example) to the fan flange screws as well as sealing the transition from flange to van around the perimeter of the fan.
3) For the OCD types
, keep item 2) to a minimum with nice clean beads (even use masking tape and tool the sealant around perimeter) and then add a third layer of protection: 3M extreme sealing tape or EternaBond Roof Seal min of 2" wide (3" is probably ideal) over the fan flange and onto the van around the perimeter of the fan.
I like the idea of Dave's alum reinf but would definitely be applying sealant between the alum and fan. Part of me is concerned with this second lap joint as a potential leak point although a 3 pronged approach would ease my concerns.
Important:
- Ensure compatibility of all products with the acrylic fan housing; the instructions have a long list of specific incompatibilities. On this topic, still waiting to here back from EternaBond
- A bit of Scotchbrite prep on van and acrylic sealant bond areas and either IPA or product specific primer ensures a top shelf, long lasting bond
- Watch product min service temps if your from the great white north. Some 3M products don't have readily published minimum service temps and Dicor has yet to send me their product datasheets. Personally I'm striving for -40oC as a min and would prefer a -50oC since -40oC is not uncommon in my neck of the woods.