What happens if I don't use undercoat?

If you don't use undercoat or primer, several problems can happen depending on surface. For bare porous surfaces like bare wood, bare drywall, or bare metal, without undercoat paint will soak in unevenly, causing blotchy, uneven color, poor coverage requiring many extra coats, and poor adhesion leading to peeling, cracking, and flaking sooner, because undercoat seals surface and provides bonding layer. For bare wood, tannins and knots will bleed through paint showing brown stains, and moisture can penetrate causing wood to swell and rot. For bare drywall, mud joints will show through and texture will be uneven. For previously painted surfaces in good condition with similar color, you may get away without undercoat and use two coats of quality self-priming paint, but finish may be less durable and coverage may be poorer. For car underbody, if you don't use undercoating rust protection, metal will be exposed to water, salt, and stone chips, leading to rust and corrosion that can shorten car life, especially in rust belt. For exterior house painting, skipping undercoat on bare spots leads to premature paint failure. So if you don't use undercoat on bare or problem surfaces, you risk uneven finish, bleed through, poor adhesion, and short lived paint job that peels quickly and requires repaint sooner.