When is paint primer used?

Is paint primer used to paint a stucco wall that is already colored? Also, for painting a wooden deck/porch that has never been painted before, would a primer be needed? Is that before or after the sealer is applied? Thanks!When is paint primer used?
1st apply a Knotting solution to all the knots on the bare wood, then apply either an all purpuse primer or an aluminium wood primer, then undercoat, then top, and a primer is a paint which seals a surface, so anything that has been previously painted and that still has paint on it you dont need to prime it, hope this helps, im a decorator by the wayWhen is paint primer used?
I'm not sure about the deck part but yes it is used when changing colour if you want the desired shade or if unsure what kind of paint was used in first place.
paint primer is used under several conditions:


when the surface is of poor quality


whet he surface is previously colored, and you are going from a darker to lighter tint


when a surface, such as a deck, has never been painted





if the surface is a deck, a sealer is not typically used if painted ( if using an exterior oil paint). sealers are usually put on after staining...
Ask your local paint dealer. Explain what it is you are doing and he will tell you the best way to proceed.
The object of a primer is to provide a good key to the surface so that the new finish sticks on. For masonary you would usually use a stabilizing fluid which will be applied after rubbing over to remove any soft or flakey areas.





The finish on the decking must take into account whether the timber has been pressure treated with preservatives as this can bleed through some finishes and ruin the appearance.





As has been said, check with a paint supplier and ask for the guidance on the best products for you situation.





DWD