Etch -vs- standard primer ?

[Weldinator]

I understand how etch primer works on bare steel and wonder if standard primer is really up to the job of sticking to bare steel ?

I ask this because on some of the paint jobs iv done on my project i have used both zinc filler primer, 90% zinc primer and standard primer on bare steel areas. I have used standard on small areas of bare steel to help stop rust recurring till the time came to paint the top coat on. I know the zinc primers will help to stick to bare steel.

If standard primer is not really up to the job then what is the point of standard primer on bare steel or is it only for top coating etch primer ?

Also can etch primer be used on top of existing paint etc ?

The area is inside the engine bay.

 

[outofthefire]

I am no expert but I thought etch was dusted on to provided a key for primer, thats how its used at work anyway.

 

[mm289]

have a read of this http://www.mig-welding.co.uk/forum/showthread.php?t=30627

Thread currently running which has a lot of useful info

MM

 

[sprayerDrew]

some primers have excellent adhesion to bare steel but need an etch or epoxy under to add corrosion protection

 

[Weldinator]

The standard primer thats still on the steel has bonded very well and i'd have thought once steel is air tight it wont have oxygen to start rusting anyway.

It's just etch will lift edges of the original paint, so the clean steel patches i have sprayed in standard primer can only really be painted with standard primer. Oh and all the areas that have been derusted, i have used a wire wheel brush so the steel surface will have a good key on it's surface.

As its only inside the engine bay a few patches here and there of standard primer wont really do any harm will it ?

 

[Chipper]

Few small patches should be OK, etch has better adhesion though because of the acid. Both are porous though, unlike epoxy.
I find wire wheels burnish the surface, so give it a quick sand before paint to get the key back.

 

[Weldinator]

So do you suggest i get rid of it all and start again ? It's a pretty dry garage well sealed and its been very dry over the time its been sprayed. I have used some filler in a difficult place to sand it down flat, i dont want to have to do it again. I have used zinc filler primer on that though.

 

[Chipper]

Should be fine, like you say small patches should be no problem. If you did want to etch, light coats should be better for not bothering existing paint, only wants a light coat anyway just to get that bite into the metal.

 

[Weldinator]

Ok cool thanks for the tips and advice guys.