Which is better, primer or undercoat?

Primer and undercoat are often used interchangeably, but they have slightly different roles, and which is better depends on stage of painting. Primer is first coat applied to bare, untreated surface to seal, bond, and create base for following coats, it has high adhesion and sealing properties but less filling and may be thin. Undercoat is second coat applied after primer and before topcoat, or sometimes used as combined primer undercoat, it builds film thickness, provides even base, improves topcoat coverage, and adds durability, often more pigmented and thicker than primer. In traditional three coat system, you apply primer, then undercoat, then topcoat or gloss. In modern systems, many products are primer undercoat combined, labeled as primer and undercoat in one, which does both sealing and building. For bare wood or drywall, primer is more important and better as first coat because it seals. For previously painted but uneven surfaces, undercoat is better to provide even base. For best results on bare surfaces, use dedicated primer followed by undercoat then topcoat. For repaint of good condition surfaces, quality self-priming undercoat or paint and primer in one may be sufficient. So neither is universally better, primer is better for bare surfaces to seal, undercoat is better for building base and coverage, and combined products are convenient.