A leaking sight window in a new built tank is certainly a dissappointment, but can be fixed properly. Locktite 290 is probably not a good idea. Looking at the technical data sheet for this product
http://www.loctiteproducts.com/tds/T_LKR_GREEN_tds.pdf, it is intended for locking and sealing metal fasteners. There is no application listed similar to fixing a proseal leak. It is a wicking product which sounds like the easy way out, but may in the end cause a lot more grief if it only fixes the leak for an air test, and then you have to pull the tank out of the airplane for the correct fix.
I'll bet that the consensus will be to pull the top off the tank and reinstall the window being extremely careful to follow the plans exactly and work carefully.
For those new and potential builders, there have been no reports of leaking sight glass windows since Van's issued the service notice to repair the original sight window design which did have leak issues. This is the first report of a leaking window that I recall being posted and it is during test of a new tank and not a leaking window in a flying aircraft. There are not a lot of 12s flying around with leaking sight glass windows. There is no evidence that the window is a leak problem that must be fixed by deleting the window.
Most GA aircraft have fuel tanks where the pilot is able to see the actual fuel in the tank prior to flight. This may be a condition of the ASTM standard that the 12 complies with. Without the sight window, this would not be possible.
The installation of a mechanical gauge in the tank looks like a substitute for the window but it really is not and it is a mechanical device (for boats) that can hang up, although that has not been reported. Just saying...
Personally, I like the sight glass. No moving parts, and you can see the actual fuel in the tank during preflight.
Sorry for the long post, but I wanted to state that the tank window is not a leak problem flying around in all these 12s and that the original problem was re-engineered and fixed by the factory long ago.
For those still in doubt about the window, do a search of the forum posts and do the research.
Tony