Skirting boards typically benefit from undercoat, though it's not always strictly necessary depending on the material and surface condition. Undercoat serves several important functions: it provides better adhesion for the topcoat, improves coverage (especially over darker or stained wood), seals porous surfaces, and can help prevent the need for multiple topcoats. For wood skirting boards, particularly softwood or previously stained boards, undercoat is generally recommended to achieve a professional, durable finish. It fills in grain and creates a uniform surface for paint application. For already-primed boards or high-quality prefinished materials, you may skip it if they're in good condition. If you're applying gloss or semi-gloss paint, undercoat helps create better adhesion and a smoother final appearance. For matte finishes, it's less critical but still beneficial. The skipping of undercoat typically results in needing additional topcoats to achieve full coverage and opacity, which can actually cost more in paint and labor time. On previously painted skirting boards in good condition, light sanding and a good primer-undercoat combination may suffice instead of full undercoating. Overall, using undercoat represents good practice for most situations, as it provides better results, enhanced durability, and often better value than skipping it and applying extra topcoats.