Pigments in some sanding sealers can be moisture sensitive and attract damaging moisture. Other pigments are meant to slough off easily and do not provide a strong cohesive bond for other coatings. McClosky TungSeal is an interior product meaning that it has no wood preservatives or water repellants in it. This particular product takes a long time to dry sometimes and applying varnish before it is completely dried may weaken it. Applying more of this particular stain to get a deeper color creates a thick film as stains go.
New wood requires sanding before application of any kinds of coatings.
Most wood comes from the mill with a glossy finish created as the wood is drawn via high pressure rollers through the planing machines. Sanding the mill glaze off allows for the mechanical. We rely on the thickness of the coating to provide sufficient UV and moisture barrier properties. If there is not enough varnish on the door, UV could be killing the undercoatings. Sometimes varnish is only placed on the face and the upper and lower edges are omitted.