We just bought a custom oak front door in June. It’s December of the same year and it’s cracking. I used tung oil to seal it. Applied 3 coats in June and again in October. Everything I’ve read about tung oil says it’s a superior exterior product because it cures. So is it a door issue? Should we contact the company? I understand doors expand with weather and humidity, but there’s are cracks large enough to see through and feel cold air coming in.
Solid wood doors with panels are to be constructed so the panels float in their groove (dado) or perhaps the panels are held in place by an applied molding. Regardless of panel construction, or other types of joinery used in door construction, the panels are not to be fastened in place. They will expand and contract. Because of this, with some makers that pre-finish their doors, the panels are finished before installation in the door. On larger panels, since the possibility of a panel shrinking so much that an edge may fall out of the dado, one single fastener is placed in the center of the panel so the movement will be divided up at both sides, instead of possibly moving all from one side.
There is a chance that any finish you have applied may have "bridged" the gap, and sort of glued the panels in place so they will not move, and therefore cracked.
If I remember right, tung oil is a great finish for furniture, easy to apply, can be touched up easily. For doors as long as they do not get wet from rain or snow melting on them, the finish should not fog up.
Cracking often comes from moisture penetration and most moisture penetration enters through end grain. So be sure to protect (seal and keep sealed) the top and bottom of the door.