When applying the initial primer to skirting, it's common for the material's surface (wood or MDF) to lift slightly - requiring sanding down before a second coat is added. This process of double priming and sanding must occur in order to acquire an even surface prior to adding the final coat.  Many manufacturers neglect this procedure during the manufacturing process, resulting in rough finished products. Keep in mind that even with double priming and sanding, a final coat of sanding may still be needed before the board is complete. Skipping or cutting corners on this step could result in an uneven finish that requires more effort to correct.