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I already found several articles explaining how to properly spray paint a metal surface, so that's not my question.

My botched attempt to spray paint a metal surface that had already been painted resulted in these strange streaks that looks like someone came in with a toothpick and ran it along the surface when the paint was wet (but of course they didn't).

enter image description here

enter image description here It was so unexpected, it got me curious. What's the cause of this? I don't mean "because I didn't sand it first". I mean, what's the physics or chemistry at work to cause this particular phenomena?

I was unable to find anything about it on the internet.

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Something is causing the layers of paint to repel away from the areas, leaving a significantly thinner coat. There are three phenomena at work.

  • surface contaminants, such as oil, wax, silicones, etc.
  • A glossy surface, which after all is designed to prevent dirt and contaminants from sticking.
  • fundamental differences in the adhesion or absorption of paint due to different surfaces.

You cure that with a three step process:

  • First, you sand it sufficiently to remove gloss, rust and contaminants like silicones.
  • Second, you wipe down with a strong enough solvent to remove oils, waxes, and the like. This solvent must not leave residue. A two-cloth cleaning is best to assure you're not just swirling the contaminants around.
  • Third, you use primer to form a surface that is consistent in regards to absorbing paint and adhesion, if those issues exist.
  • I would also ask if this seems right to you: There were tiny invisible to me scratches on the original surface. That exposed the metal below. And the paint didn't adhere along the lines of those scratches. Is that something you've encountered in exactly that way (geometrical paint repellant lines caused by "invisible" scratches or dings)? – toddmo Jul 30 '17 at 16:47
  • Yes. The cracks either wick up the paint, or contain contaminants a simple wipedown is unable to remove. Simply a more porous surface wicking up paint is enough. – Harper Jul 30 '17 at 17:12

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