From what I've read the Lexan will likely eventually need to be replaced at some point in the future ... no matter how careful one is about not getting fuel or chemicals on it. The Lexan seemingly eventually takes a toll from the stress created by the bends required to fit the shape of the RV-12. There are many threads here on VAF about the Lexan developing cracks. Granted, most are seemingly fuel related ... but not all.
My kit didn't come with the new plexiglass window but it had just been introduced .... so I didn't even bother with sealing the Lexan window with proseal during install knowing at some point in the future, I'll just make the switch to the plexiglass window and be glad I did.
My suggestion ... DO NOT use the Sikaflex because it will likely make replacing the Lexan window a real bear of a job when you need to replace it .... and it is very likely that you will need to do so years down the runway.
RV-12 Wings, Empennage, Fuselage, Finishing, Avionics and Powerplant kits all completed
Dues paid until September 2019