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I'm using spray gun with automatic reciprocator system to coloring stainless steel sheet with some translucent lacquer. However, I found the color is unevenly sprayed and can't figure why is it uneven.

I've tried several ways including:
1.Different distance between spray gun and the steel sheet.
2.Different angle of the spray gun.
3.Moving speed of the spray gun.
4.Spray times and spray volume.
5.Air pressure of the spray gun.
6.Pressure of the lacquer.
7.Different temp and humidity.(64F~85F, 3X%~7X%)
8.Different spray gun.(Iwata LPA-200 and other locally manufactured spray gun.)
9.Cleaning the gun.

It looks like blushing but I think it's not blushing because the following reason:
1. Situation gets better when I tune the air pressure higher while it is believed to be worse if it's blushing.
2.I also tried different temp and humidity.(64F~85F, 3X%~7X%)
3.I've tried to heat up the steel sheet to about 85F-90F before spray but still in vain.
enter image description here enter image description here enter image description here enter image description here Is there anybody know the reason?


Update

  1. I've cleaned it with Acetone before painting.
  2. For the plate surface, I'm using hairline or "fine bead blasting" plate, not mirror(No.8) plate.
  • Maybe a silly question, but why are you doing this? – Nothingismagick Aug 29 '18 at 13:07
  • And are you really only coating once? I always do at least three coats... – Nothingismagick Aug 29 '18 at 14:38
  • @Nothingismagick Thank you, I'm trying to paint while not covering the pattern(Like hairline or so). I've tried coating once and also multiple coats but still in vain. – nosnhoj Sep 4 '18 at 3:22
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Try scouring the stainless steel gently with Comet (yes, bathroom cleanser), maybe with a sponge.

You can substitute a similar brand, like Ajax, but it must be chlorinated. Avoid Bar Keeper's Friend or Bon Ami as I haven't tried those.

You might want to keep your strokes in line with the grain of the stainless steel if it has any, and you probably want to experiment on a piece of scrap.

Rinse with hot water after scrubbing and wipe dry, but don't touch with bare skin. The hot water speeds air drying. Gloves can help with handling.

You can test the surface with a few drops of water. If it beads up, it didn't work. The water should "sheet" and "wet" the surface wherever it goes. The paint works similarly and will normally form a smooth surface tension over the entire surface treated this way, rather than in spots or splotches.

The scouring "activates" the surface (for a few hours or maybe overnight), increasing the molecular attraction of the applied liquid. Because the liquid molecules are attracted to themselves (which is what causes droplets to form and beading to form on a surface), the surface attraction needs to be greater than that inherent in the liquid. Any oils on the surface inhibit that attraction and cause beading. There are enough hydrocarbons just floating around in the air to coat a surface in a few hours with enough to cause beading. So don't leave it sitting around after cleaning, paint as soon as it's dry if you can.

As well as increasing the surface area by texturizing the surface just a little, the chlorine in the scouring powder breaks down the surface hydrocarbons with an effectiveness that solvents can never match. A solvent dilutes the hydrocarbons (oils) but all it takes is a layer one molecule thick to disrupt the surface activity and the solvent always leaves some behind when it evaporates.

  • Thank you, Scanny. I'll try it. However, I'm not living in USA, I'll try to get some Comet first. :D – nosnhoj Aug 28 '18 at 0:48
  • Hi, Scanny. I've got Comet and tried what you said but still in vain. I've also tested the surface with a few drops of water and there is no beads up. Is there any suggestion? Thank you very much! – nosnhoj Oct 23 '18 at 3:39
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Are you cleaning the stainless steel before you paint it? Finger oils left on the surface by general handling can hold onto more lacquer than the areas which were never touched.

Also, you might want to lightly sand the steel with a high-grit-count fine sanding paper just to slightly abrade the perfectly smooth surface. That might allow the lacquer to grip the metal better, allowing for a more consistent and longer lasting coating.

  • Thank you Henry, I just forget to say that I've cleaned it with Acetone before painting. I'll update my question. As for the surface, I'm using hairline or "fine bead blasting" plate, not mirror(No.8) plate. So it's not smooth surface. – nosnhoj Aug 28 '18 at 0:36

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