Why does a wall need primer and putty before the final coats?

Plastered walls look finished, but at close range they are rough, porous and slightly uneven. Final paint is a thin film that follows whatever is underneath it.

Putty, also called wall putty or skim coat, is a fine cement-based or acrylic-based filler applied in thin layers and smoothed. It evens out pinholes, trowel marks and minor undulations. It also provides a uniform, light base, which helps colours look true. Done well, it is the main reason a wall appears flat under side light.

Primer is a thin coat that does different work. It seals the porous surface so the topcoat is not absorbed unevenly, helps the paint to bond, blocks alkali that can attack the finish, and can resist dampness or stains, depending on the type. Different surfaces need different primers: one for new plaster, another for putty, another for metal or old gloss. Using the wrong primer can be as bad as none.

What goes wrong without them:

• Patchy colour, since absorbent and sealed spots take paint differently.
• Visible ridges and hollows, especially under tube lights and low sun.
• Early peeling, flaking or blisters when the paint cannot grip.
• More topcoat consumed to cover the same area, which cancels the saving.

Expect the contractor to allow drying time between layers and to sand lightly between them. If one day produces the whole job on a wall, the layers were probably rushed. Treat the cost of these layers as part of the finish, not an extra.