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Before painting a small item (think of a plastic model), I wash it in soapy water to remove oily fingerprints etc. Most commonly I use diluted dishwashing liquid. From experience, washing is important, especially for non-porous surfaces like plastic or metal and paints with poor adhesion (like acrylic).

The question is: how important is it to completely wash the soap residue off the surface for the paint quality/adhesion?

I'm not talking of anything obvious: the dried surface always looks perfectly clean. But if I don't flush it well and smear a wet finger over the surface, I might feel slight soapiness. Is it bad for the paint?

If we need to narrow it down, let's say the paint is acrylic (alcohol-based) and applied with an airbrush (the goal is even coverage without strokes).

Consider that it may be difficult to wash the soap off completely: often the piece is too fragile or too big to be flushed with running water, and some parts of it may not be water-proof. Usually I have to apply both the detergent and clean water with a soft brush.

Theoretically, dishwashing liquid is a surfactant, so if anything, it should help the paint (unless it has a specific reaction to the dye). But maybe someone has direct experience with this.

I didn't notice anything wrong myself, but I do try to wash it off well, if not perfectly. This is a tedious process. Maybe I'm overzealous...

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  • Just speculation, but while a thick coating of soap might be a problem, a microscopic thin layer I would expect to diffuse into water- or alcohol-based paint and not affect it once it dries. Haven't tested it, though.
    – fixer1234
    Aug 25, 2021 at 17:40
  • I suspect the reason you haven't had trouble is mainly that the hardest areas to rinse thoroughly are also the least likely to get knocked or abraded, so slightly reduced adhesion matters less there
    – Chris H
    Jan 21, 2023 at 8:58
  • It's also possible the detergent residue was on your finger and not on the surface. May 26, 2023 at 14:43

3 Answers 3

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It is very important to completely wash the soap residue off your surface before painting. Acrylic paint can very easily peel off of plastic, so by leaving a thin soap layer you're only increasing your chances of the paint peeling. Afterward, rinse the plastic in just water no soap with a scrub brush. Then, let it completely air dry (should take anywhere from a few hours to overnight).

Things I would recommend to prevent paint peeling.

First, using a fine-grit sand paper over your plastic before painting. That way, the plastic won't be as smooth and the paint will adhere more.

Second, using a paint primer, and then letting it completely dry before you paint your final color. By adding a primer, you're increasing your odds of a perfect application. Another benefit to a primer is that depending on what color the plastic and the paint color, you can make your paint coat more vivid by paint on a white primer first.

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    You just state that it is "important". That's sort of common wisdom, but my question is why is it important, and/or have you had actual observations that acrylic paint adheres worse if soap is not completely washed off. (Remember we are talking about absolutely invisible layer, not something obvious). I also mentioned that the issue arises when the parts are too delicate to use scrub brush and sanding (or even washing!) Yes, primer is a norm for acrylic on plastic, but sometimes it's necessary to wash the primed piece (after sanding or handling without gloves), so the problem is the same.
    – Zeus
    Sep 14, 2023 at 2:02
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It would depend on the type of detergent, and the type of paint used.

For example, an alcohol or oil based paint won't take evenly to a surface that has detergent residue on it. You may experience blotching, pealing, or uneven drying.

Standard acrylic paint would take more or less normally so long as the surface was dry.

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    And this answer is downvoted for what reason? Jan 21, 2023 at 13:09
  • It depends is not an answer.
    – rebusB
    Nov 28, 2023 at 19:18
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It is very important, because:

  1. Soap residue can create a barrier between the paint and the surface, hindering proper adhesion. Paint needs to bond directly with the surface to adhere well.
  2. Soap residue can act as a contaminant on the surface. The remained thin film can interfere with the paint's ability to bond with the surface and may cause the paint to adhere unevenly or poorly.
  3. Paint adhesion is crucial for long-term durabilityof the finish.
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    Does it create a barrier, though? Does the soap form a layer between the surface and the paint? Or does it mix, and, if so, is that necessarily bad? While your answer contains a lot of truisms, I'm not sure it's specifically helpful.
    – Joachim
    Nov 6, 2023 at 22:39

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