I'm refinishing solid pine wood doors in my home, the doors are original from 1940 when the home was built. After stripping multiple layers of paint, I found a few panels separated on some doors (just 2 out of 10 doors total). Here's one about 3/16" thick. Before staining/sealing, is there a simple way to fix this to avoid separation in the future? I live in N.Y., so I'm concerned about seasonal temp changes.
UPDATE, side photos showing joint gap
I have had some similar projects. On occasion, I have been able to force glue into a joint with a glue syringe and clamp the joints together. The key to this method is cleaning out the joint with a putty knife or something similar and clamping it tightly.
The method that has been most tried and true for me is, unfortunately, a bit more involved. Those old solid wood doors are great and ridiculously expensive to replace with the same quality so, in my humble opinion, it is worth the effort. Over the last 70 years the expansion and contraction you are concerned about is most likely what has separated the joints. I would expect a door from this era has nice big mortise and tenon joints. Work the joints apart gently until you have separated all the stiles and rails. Clean all the old residue off the joints and use thin pieces of wood to fill in gaps to get the joints to go back together tightly if necessary. Coat both the mortise and tenon sides with a good wood glue to and tightly clamp the doors back together. Hopefully, this will restore the original integrity of the door by maximizing the glue surface of the joints as originally intended and perhaps they will last another 70 years.
Is the joint still solid? That is, if you grab the two pieces on either side and shake them, is there any movement? If it's solid, then it means that the gap is just a result of shrinkage, and it's merely a cosmetic problem. Just slap some wood filler in there and sand it down. But if it's actually separating, then you'll have to re-glue it (see WoodAirGrille's answer).
