silly engine painting question

[mattyo]

Is it necessary to use primer on a sand blasted surface before painting? My interpretation of primer has been that it prepares a good surface for the paint to stick to, but it seems that a sand blasted surface is already set for a paint job so the primer is not needed. But I want to get your thoughts on the topic.

[Buicknick72]

If it is specific engine paint, I beleive it is designed to be used without primer. I'd read the paint can, it should say.

I've never primed engine parts before painting them

[atdawnweride]

I prime everything before paint.

[mattyo]

Well VHT specs out a procedure of (http://www.vhtpaint.com/products/engineenamel/):
1) strip fast
2) prep spray
3) primer
4) color
5) clear coat

But that assumes you're doing all the old paint stripping and surface prep with their product which is completely different than blasting the surface. So I'm not sure...

[mattyo]

Originally Posted by atdawnweride

I prime everything before paint.

Well, so what is the fundamental purpose of the primer? I want to learn the facts and reasoning behind the primer so I can make an educated decision about how to proceed. For example, does the primer have some better bonding properties than colored paints to get a good bond to the metal surface before painting? Does the primer have some graininess to help the paint stick better to the primer than it would to bare metal? I should google this subject but engine primer seems like it might serve a different purpose from regular primer. I don't know...

[roscoenr]

Originally Posted by mattyo

Well, so what is the fundamental purpose of the primer? I want to learn the facts and reasoning behind the primer so I can make an educated decision about how to proceed. For example, does the primer have some better bonding properties than colored paints to get a good bond to the metal surface before painting? Does the primer have some graininess to help the paint stick better to the primer than it would to bare metal? I should google this subject but engine primer seems like it might serve a different purpose from regular primer. I don't know...

The primer usually has an acid in it that helps it bond, or etch, to the metal surface. Then the paint will have a good surface to stick to.

[corvin]

You can paint directly if the paint is specially for aluminium.If not I use primer for aluminium and chromed,nickeled surfaces.
If You paint on bare aluminium with ordinary paint it is not going to last long befor start to peel.

[Roaring_Shadow]

Originally Posted by roscoenr

The primer usually has an acid in it that helps it bond, or etch, to the metal surface. Then the paint will have a good surface to stick to.

This. Primer is basically a bonding agent. In simple terms, its like putting glue (primer) on paper (metal) for glitter (paint).

[Buicknick72]

If the manufacturer says to use a primer, I'd use it. Especially if it is your bike engine that is out on display.

[atdawnweride]

Originally Posted by mattyo

Well, so what is the fundamental purpose of the primer?

As mentioned, the primer acts as a bonding agent. I don't have the scientific evidence or explanations in front of me, but I do know from experience that paint on a primered surface will look better and last longer than on an unprimed surface.

I learned a long time ago that skipping steps and cutting corners during paint prep only costs more time and money when you have to repeat the process much sooner than you should have to.