My 100-year-old house has a wooden front door made of thick wood ribs interspersed with 8 wood panels, each about 10" high, 8" wide, but only 1/8" or 3/16" thick. Perhaps the door originally had glass instead, then someone replaced glass with cheap, thin wood panels?
Anyway, there's a thin (1/16" wide) but ugly crack running top-to-bottom (10" high) in one panel. From the inside I even can see a little bit of daylight on the other side on bright days, although on rainy days the crack closes up as the wood swells.
I've sanded, patched, and repainted the inside of the crack once before, using lightweight spackling compound on the inside only, but I was lazy and didn't do anything to the outside of the crack that's exposed to the elements. So after a few months of rain and hot sun, the the crack reappeared.
Both inside and outside are painted (Inside: latex, outside: not sure.) The crack on the outside is barely noticeable because the exterior side door is black, but the inside is white and the crack is very noticeable. So mostly I care about fixing the inside and want to do as little as possible outside, as long as the crack doesn't get more visible outside.
Any advice for how to patch this to withstand the elements on the outside and look good on the inside? I'm looking for advice about patching material (or materials if I have to use something different outside vs. inside) and technique (e.g. outside first? inside first? anything special to worry about given that inside and outside are only an eigth of an inch apart?)