So I'm in the middle of building my tanks and now I'm to the part where you close it up for good and so I was wondering if there is something that I can do or need to do to test for leaks before closing it up?
A thourough visual inspection for voids in the fillet seals adjacent to the end ribs and fuel fittings is about the only useful "check" you can do. Filling the tank with water to leak check it at this point would only identify leaks that would be easily seen by performing a visual inspection, and is therefore a waste of time and effort.
You will want to thoroughly clean and prep the area as usual. You may want to remove any extra cured sealant from the seam prior to applying new sealant and then make sure the new sealant adheres to the aluminum skin on either side of the seam. I am assuming you will be applying new sealant to a seam. I would make attempts to insure that the new sealant is making a solid contact with the aluminum and not just to any cured sealant.Will New Sealant stick to sealant that has hardened? Do I need to do anything to the hardened sealant before applying New sealant on top of it?