Prime on metal, is it really needed?

Old 05-15-06, 12:04 AM
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Prime on metal, is it really needed?

Hey all,

Most DIY instructions say to prime a metal surface before painting it, that's what I read here while searching the forum and most elsewhere on the net.

I am currently looking into painting my motorcycle fuel tank, and when I took the old paint off I noticed it has no prime layer. That's just the way it was delivered from Yamaha.

So .. why didn't Yamaha prime the surface before painting? and why should I prime the surface before I do the painting?

Just curious ..


Old 05-15-06, 05:41 AM
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I'd be surprised if there's no primer, but I suppose it's possible
Factory auto grade finishes are often electrostaticly (sp.?) applied, and can be extremely thin
I wouldn't be surprised if the primer is molecules thick

Regardless, especially when not in a factory situation, a primer is needed

Paint in itself makes a good topcoat, a durable finish, nice looking, shiny, but doesn't stick as well as it should because the properties that make a good topcoat don't make it sticky
Primer sticks great, but makes a lousy top coat
It attracts dirt and moisture and doesn't resist chipping...and looks dull

The exception would be powdercoating
Not a paint, but a solid, a powder that is "shot" onto a bare metal piece
No primer is used
The piece is then baked in an oven
This helps the finish adhere and cure

It wouldn't be too surprising if your tank was powdercoated, just that if it was, the coating came off so easy
It's usually pretty tough stuff
I suppose a factory powdercoating (if they do it) would be pretty thin compared to a powdercoater's coating
Old 05-15-06, 06:07 AM
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Another benifit of priming automotive sheetmetal is the primer will highlight any existing defects so you can fix them prior to the finish coat. Also the primer will sand better than the metal allowing you to end up with a finer finish.
Old 05-15-06, 09:14 AM
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Also if rust is present on a metal surface even in microscopic amounts the right type of primer will block it from growth or reoccurring.It is very difficult to counter rust through prep work alone....doable but most people wouldn't do enough....and topcoats will not stop rust on their own long term.Paint manufacturers play with the truth because a coat of paint technically stops rust.....but only for awhile.
Old 05-16-06, 04:55 AM
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Cheers guys.

It's always best understanding all the small details. Your answers really helped.

(BTW, only now noticed that I posted the question under 'Interior Home Decorating', sorry for that).

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