How can I fix a hairline crack in a thin wood panel of an exterior door?

Question

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?)

Answer 1

Lightweight spackling compound is not suitable for exterior use. Even if painted, water will effect it greatly. In addition, it is prone to cracking as it is brittle and probably will not withstand opening and closing of the door (vibrations).

Wood filler or wood glue may work to fill the crack and would be better for exterior use. The glue is probably more flexible than the filler and may be better for filling the crack. I've used the wood filler before, but usually to fill nail holes or fill in small cracks. For a few bucks and some sanding, it may be worth a go round.

Another option would be to caulk the crack. Since caulk is flexible, it will resist cracking better than the wood filler. Both caulk and wood filler would be paintable. I don't think you can stain caulk, but I believe the door is painted on both sides.

The other option I mentioned in the comments is to replace the panel/door. The panel is most likely held in by surrounding trim, so if that can be removed, just the panel could be replaced.

Another option would be to get a very thin piece of veneer wood, cut it to the dimensions of the panel, and then glue it on top of the existing panel. Since you cannot see the crack from the outside, this could be done on the inside and then painted to match.