Uneven sealer application is a common concern, and there are a few possible explanations for why it happens.
The most frequent cause is painting over a dark-coloured wall. A single coat of standard white water-based sealer simply doesn't have enough opacity to cover dark shades evenly — the underlying colour bleeds through, creating a patchy, inconsistent appearance.
If this is the case, request a grey sealer from your nearest Nippon Paint dealer shop. Grey sealer is specifically formulated for better coverage over dark-coloured walls, and one coat — applied without any further dilution — is sufficient for a smooth, even finish.
As a general rule, water-based sealer should always be applied undiluted. Adding water reduces its opacity and bonding strength, which can contribute to uneven coverage and poor adhesion of the topcoat.
If you're unsure which sealer is right for your wall condition, speak to a consultant at your nearest Nippon Paint dealer for personalised advice.