I want to better weatherproof an old shed that has barn wood siding. Most of the boards have a gap between them, and these gaps can be as wide as 3/8" although most are more like 1/4". The boards are about 3/4" thick and are run vertically. and they constitute entire wall structure (i.e. there is no internal finishing, it's just sided and that's what you see both inside and out).
I would like to seal this thing against Ohio weather. What sort of filler or caulk (or ?) should I be looking for to fill that kind of gap? And how do you go about applying it when you can't get at both sides of the board at once? As in, if I apply caulk, what's to keep that caulk from just pushing through the gap and out the other side?