Definition - What does Transition Primer mean?
A transition primer is a type of coating used to bond separate and incompatible coating layers, such as a primer and a finish coat. This product allows multiple layers of coatings, which may not work well together, to be used in the finished coating system. Therefore, the transition primer must be compatible with the coating layers it is attempting to join.
Corrosionpedia explains Transition Primer
Transition primers are frequently used in the furniture industry. Before furniture can be repainted, the existing varnish must first be removed. However, while sanding can remove most of the varnish, leftover alkyd or urethane can still be present. These compounds can affect the adhesion of the newly applied paint.
Therefore, in this case, a transition primer is applied over any existing alkyd or urethane to provide a uniform and compatible surface for the topcoat.