What causes “toothpick streaks” when spray painting already-painted metal

spraypainting

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).

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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.

Best Answer

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.
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