Is undercoat really necessary?

Undercoat, also called primer, is really necessary in many situations for durable professional finish, though you can sometimes skip it in limited cases. Undercoat is necessary when painting bare surfaces like bare wood, bare drywall, bare metal, or masonry, because it seals porous surface, provides adhesion, prevents paint from soaking unevenly, and blocks tannin or rust bleed. It is necessary when painting over stains, dark colors with light colors, glossy surfaces, or repairs with filler, as it ensures even color and prevents peeling. For example, painting bare wood without undercoat leads to blotchy uneven finish and poor adhesion. Painting over dark red with white without undercoat needs many coats and still shows through. For previously painted surfaces in good condition, clean, dull, similar color, you can often skip separate undercoat and use two coats of quality self-priming paint, as modern paints have primer in them. So undercoat is not always strictly necessary for repaint of good condition walls, but it is necessary for bare, damaged, glossy, or color change jobs, and it improves durability, coverage, and appearance, saving topcoat paint in long run. Professional painters almost always use undercoat for best results.