Re: Wood Siding Joint Management
Yes the Abatron putty epoxy is rather brittle. We always cut the joints so the woodepox is not bridging two different components or the joint (esp a 90* one) will cause it to crack. But it's great for work that needs a like-new smooth or moulded surface.
The ART Flextech is a gel. It remains quite flexible; the downside is that it's hella hard to feather out or shape (compared to woodepox). I have been using the Abatron stuff since 1986, and only in the last 5 or so years begun to use Flextech. The other reason for my recommendation is that the first step of the ART system (as with Abatron) is the penetrating/sealer step, which makes it possible for the gel to deeply bond with the wood fibers.
Since you were asking about siding butt joints, I'm hearing "end grain" and any caulk job on end grain will only last until that wood gets wet. No sealant can stick once the wood's damp. The penetrating epoxy makes this a non-issue, and the sealant will continue to seal.
The lexel/geocell type flexible, paintable sealants are also good partners after the end grain has been sealed, but they are completely unworkable after drying, and the closest you can come to giving them any kind of shape is to work them with a thinner soaked rag, etc. And any subsequent fillers added are in real jeopardy of popping loose. Up high and out of sight they are invaluable.
Remove not the ancient landmark, which your fathers have set.