I have thought about this problem for months, Canabus.
There are three gaps that I need to seal
1. the sub-deck/main deck
2. lower superstructure/deck
3. wheel house/lower superstructure
I think I have found a liable solution: Loctite 5910
It is a special silicon that is meant for automotive motor parts sealing needs.
When you use it against the producer’s intention, it effectively becomes a liquid seal that, once cured, feels quite like rubber - not silicon. When I found this one, I didn’t think much about using a more common sanitary silicon the same way - maybe it works, too.
I have ”innovated” to find a solution for applying the silicon. I got it now for applying it along a straight edge (1st pic). But corners are still a problem (2nd pic). The self-made tool is shown in the 3rd pic. With practise, I may learn to run the applying tool around a corner - here masking is especially important.
CAUTION 1: remember masking (remove while the silicon still wet). I forgot it and caused a mess that can not be totally removed (it may show in the 2nd pic).
CAUTION 2: only use silicon product in your build after everything is painted and finished. Or be _extremely_ careful = brave. I forgot this rule only for the split second that it took to try the removable camera platform on the roof of the lower superstructure. Now I keep my fingers crossed when the time to paint the said roof comes! (the camera platform is the first one that I applied the ”liquid seal” under). Even if the seal-silicon had cured, there may be something left to make anything it touches unpaintable.