This can depend on the colour and type of finish that's desired. Wood stains such as Manns Classic Pine Stain are a great way to stain interior doors as they offer the widest range of colours and can often be intermixed (from the same product range) to achieve many thousands of wood tones and shades. Water-based and solvent-based liquid stains must be over-coated and sealed into the wood with a clear wood oil, varnish or wax. There are a wide range of pigmented or coloured wood oils, varnishes and waxes that both colour and seal the wood in one which are also ideal for interior doors.