Scott wrote:
On Sat, 17 Oct 2020 18:40:50 -0400, Paul
wrote:

Scott wrote:
A quick question for the experts: I want to paint a steel garage door.
Most of the old paint is intact but some is not. I have given it a
fairly heavy rub down (power tool and wire brush). I have a tin of
primer available. Should I prime the whole door for consistency or is
it better just to spot prime the areas where the paint is missing? The
time involved is not an issue. I just wonder whether paint - primer -
paint is asking for trouble or does primer stick to anything?
Just a quick question.

Is the steel, zinc galvanizing ?

I think it's galvanized. There is no sign of any rust.
If so, that changes the nature of the question.

It's a lot harder to get paint to stick to zinc,
and that's why that question has a different answer.

If the material is just plain shiny flat steel, with
excessive rust capability as a side effect, then the
paint question is a different one. Where the paint
has come off, would have rusted, if it was plain steel.
With a galvanized layer, it won't rust (at least not
right away), but on the other hand, ordinary paint
will not adhere well.

This was why I thought about primer.

Try keywords like "wash primer".

I've not used any, because I'm the guy with the galvanized
surfaces with no paint on them :-) I can tell you with
authority, that regular paint lasts around a year,
before the lack of adhesion makes it come off again.

Something is required to get a "tooth" on the material.
Otherwise, with regular paint, the zinc reacts with the
organic content a bit. The purpose of the primer, is to
not allow that reaction, and provide some isolation.

The wash can be acidic, as one means of building a primer.
And a concern would be if the acid part bleeds through
the next layer of paint or not. And the primer has to
cover the metal part, as you don't want to be seeing any
metal where the regular paint goes on top.

Paul