I already tried to seal the right rear tube with one bottle of Polymarine Sealflex, but I was badly disappointed.
It never sealed anything
not even the smallest pinholes.
Even today when I inflate the tube, I can see the milk bleeding through the pinholes, just like water.
Maybe the non-boat specific liquid you suggest is the better choice. I should give it a try.
I also tried Devcon Flexane 60L on the outside of the tubes.
(Quite expensive PU coating. Some guys even repair airplane tires with it) Really tough material, but no chance to seal everything properly in deflated situation.
With inflated tubes, of course, it blows bubbles.
Maybe it would work better in the tube. But no idea how to get this into the tube, it‘s to tenacious to fill it through the valve……