We are refinishing our old 5-panel wooden doors throughout the house (they are currently painted with numerous layers of paint). We use a heat gun to get most of the paint off, and then follow up with a chemical stripper to get the rest of the paint especially along the inlays where the trim meets the inner panels.
A few of the doors, we only used the heat gun. We then primed the surface with water-based primer and then latex paint. Now, on all of the doors we’ve done, the paint is starting to crack, or separate, in spots, right where the trim meets the inner panels. Usually this starts to happen in less than 3 weeks after we’ve finished the door. We have a few doors that only have primer on them and even that is starting to crack right along the trim line, as in the other doors. We have tried two different primers, but both of them are Zinnser and both are water-based (one is a primer and sealer). I know the tops and bottoms of the doors are not sealed in any way and wonder if this is allowing moisture to get in, causing the wood to expand and contract?
This is common and expected for this type of door. The panels are set into the frame and are loose to allow expansion and contraction. The primer is cracking along this line because there is movement of the panel.
Don't try to glue or seal the panels to the frame. Cracking of the panels will result from them not being able to move.
Since you have nice stripped clean doors spraying is the best choice for primer and paint application. If you can't spray try to apply a thin controlled coat near the gaps, this will look better than filling the gaps and allowing them to crack.