Primers and undercoats

Primers, be they metal or wood primers, are formulated to be in direct contact with the raw substrate. When preparing a surface for repainting, one of the requirements is to scrape off loose or flaking paint . When the prep work is complete, and the substrate has been exposed in discrete areas, that particular area needs to be spot primed with the appropriate primer before recoating. Many times we see metal primers being used over the entire surface of previously painted metal or wood objects, before the object is repainted. Anti-corrosive pigments, used in metal primers, need to be in contact with the metal substrate for them to be effective. Similarly, oils in wood primers need to be in contact with wood for them to lubricate wood fibres.
Plaster or Masonry Primers are alkali resistant, breathable coatings designed to penetrate and bind the upper layer of a plastered substrate. Often we see contractors using "Universal Undercoat" in place of Plaster or Masonry Primers, often with disastrous consequences. Universal Undercoat's are not alkali resistant, and should only be used over primed surfaces. They contain neither anti-corrosive pigments, nor wood-fibre lubricating oils, making them unsuitable for use directly onto metal or wood.