A can of floor and porch paint says "do not prime horizontal surfaces".
The obvious question is, how are horizontal surfaces different from vertical surfaces?
But my real question is, what are the consequences if someone does prime the wood decking on their porch?
(Other than having to ask embarrassing questions like this.)
Porch and floor paints are formulated to work well on floors and are self priming. Very few primers are rated for floor use, so what you would have is a "bad" primer layer followed by proper floor paint which would give inconsistent results.
Consequences would most likely be chipping and peeling paint because the primer was too thick, too soft, or just didn't adhere well to the surface. Water resistance might also be affected.
For floor paint (and really most painting) surface prep is very important. You can't rely on a primer to cover up issues - you need to prep the surface exactly how the porch paint describes.
