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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
    Commented 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
    Commented Jan 21, 2023 at 8:58
  • It's also possible the detergent residue was on your finger and not on the surface.
    – Little Girl
    Commented May 26, 2023 at 14:43
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    Is there any reason (besides price) why you do not wash it with alcohol or acetone (or something similar), which can remove quite a lot, and then evaporate "without" residue? (without = depending on their quality).
    – virolino
    Commented Aug 6 at 6:35
  • @virolino, acetone is usually no-go as it is too aggressive for many surfaces (e.g. plastic or anything already painted). Alcohol sometimes can be used, indeed, but in many practical cases water + soap works better, particularly when there is lots of loose dirt (like sand dust), which needs to be "washed off". Even alcohol needs to be washed off or rubbed off in order to remove dirt and not just spread it around, so you need a decent quantity (price!) Finally, alcohol is not good over (many types of) acrylic paint. So yes, point taken, but for the purposes of this Q we are talking about soap.
    – Zeus
    Commented Aug 6 at 8:44

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
    Commented Sep 14, 2023 at 2:02
  • @Zeus: you kind of contradict yourself. Either you wash in soapy water and then need to remove the soap, OR the thingy is too delicate "even for washing!". Confusion of the highest order, here, I guess :)
    – virolino
    Commented Aug 6 at 6:27
  • @virolino, everything is doable, but it all requires effort. The question is whether it's worth it. The parts are often delicate/complex enough that they can't be simply put under a running tap or submerged, so I might spend 5 minutes applying soap with a soft paint brush. Washing it off completely with the same method would take at least 10-15 minutes, and for what? Maybe a simple 5-min "single coat" wash would suffice, or perhaps none at all? I know people who barely rinse dishes and they dry with dish soap bubbles on them. Yikes, but is it actually harmful? Same thing...
    – Zeus
    Commented Aug 6 at 7:58
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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?
    – Aaargh Zombies
    Commented Jan 21, 2023 at 13:09
  • It is a generalized problem on all (or most) StackExchange sites to have downvotes just because they are possible and anonymous. You have to learn to live with it, unfortunately :( Since moderators are complacent with it, we cannot do anything.
    – virolino
    Commented Aug 6 at 6:30
  • @virolino This isn't something moderators decided upon and can control. But it is annoying all the same. Most of the time you can deduce why something was downvoted, but not always. The SE network system is appealing and fun to work with, but flawed.
    – Joachim ♦
    Commented Aug 6 at 7:00
  • The Q specifies standard dishwashing liquid and alcohol-based acrylic, if you want to be specific. What is "standard acrylic"?
    – Zeus
    Commented Aug 6 at 8:52
  • @Zeus, from context, standard acrylic model paint. Such as GW or army painter, maybe? Standard as opposed to something like metallic or a speed paint?
    – Aaargh Zombies
    Commented Aug 7 at 7:07
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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 ♦
    Commented Nov 6, 2023 at 22:39
  • @Joachim: I think that the question itself invites to truisms: "If I can feel the soap with my finger on the surface, will it affect the result of painting?" Of course it will. That is the purpose of cleaning, to remove (dirt), not to add (soap). I think that the question is badly formulated, if at all appropriate. Additionally, OP even mentions in a comment that the piece to be painted might not even be suitable for washing.
    – virolino
    Commented Aug 6 at 6:37
  • @virolino But on the other hand, the main question we can take away from it, "will even the slightest film of washing liquid (detergent) be bad for the subsequent paint layer?" (where "bad", while not a particularly clear word, can be easily interpreted as 'having the quality of having a detrimental effect on longevity and/or appearance'), is a good, appropriate, and important question.
    – Joachim ♦
    Commented Aug 6 at 7:01

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