Primer allows the paint to adhere better by providing a uniform surface, and will help it last longer. It can also seal a porous surface, block out stains from bleeding through the topcoat, prevent rust formation on iron and steel, and prevent tannin bleed on wood.
Use the following guide to help select the right primer for your surface:
Problem Solving Primers Chart
Drywall: Use Zinsser® Bulls Eye 1-2-3®
Masonry: Use Zinsser® Bulls Eye 1-2-3®
Metal: If surface is rusty, remove loose rust and use Stops Rust Rusty Metal Primer. If surface is clean or lightly rusted, clean surface and use Stops Rust Clean Metal Primer.
Plaster: Use Zinsser® Smart Prime®
Plastic: Use Zinsser® CoverStain® or Zinsser® B-I-N®
Tile: Use Zinsser® Bondz®
Wood: For unstained bare wood, use Ultra Cover 2X Primer. For painted wood, no primer is needed.