<p>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).</p>
<p>Things I would recommend to prevent paint peeling.</p>
<p>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.</p>
<p>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.</p>

CC BY-SA 4.0
2023-09-14T02:02:23.570Z
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
CC BY-SA 4.0
2024-08-06T06:27:30.003Z
@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
CC BY-SA 4.0
2024-08-06T07:58:48.600Z
@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
CC BY-SA 4.0
2023-09-13T15:49:41.473Z
2023-09-13T15:49:41.473Z
Kkell1150
<p>It would depend on the type of detergent, and the type of paint used.</p>
<p>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.</p>
<p>Standard acrylic paint would take more or less normally so long as the surface was dry.</p>

CC BY-SA 4.0
2023-01-21T13:09:04.290Z
And this answer is downvoted for what reason?
Aaargh Zombies
CC BY-SA 4.0
2024-08-06T06:30:26.533Z
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
CC BY-SA 4.0
2024-08-06T07:00:24.587Z
@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
CC BY-SA 4.0
2024-08-06T08:52:47.670Z
The Q specifies standard dishwashing liquid and alcohol-based acrylic, if you want to be specific. What is "standard acrylic"?
Zeus
CC BY-SA 4.0
2024-08-07T07:07:03.627Z
@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
CC BY-SA 4.0
2023-01-20T19:14:30.757Z
2023-01-20T19:14:30.757Z
Aaargh Zombies
<p>It is very important, because:</p>
<ol>
<li>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.</li>
<li>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.</li>
<li>Paint adhesion is crucial for long-term durabilityof the finish.</li>
</ol>

CC BY-SA 4.0
2023-11-06T22:39:04.550Z
*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
CC BY-SA 4.0
2024-08-06T06:37:45.247Z
@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
CC BY-SA 4.0
2024-08-06T07:01:42.320Z
@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
CC BY-SA 4.0
2023-11-06T10:08:33.197Z
2023-11-06T10:08:33.197Z
Art History
<p>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 <strong>dishwashing liquid</strong>. From experience, washing <em>is</em> important, especially for non-porous surfaces like plastic or metal and paints with poor adhesion (like acrylic).</p>
<p>The question is: <strong>how important is it to completely wash the soap residue off the surface</strong> for the paint quality/adhesion?</p>
<p>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?</p>
<p>If we need to narrow it down, let's say the paint is <strong>acrylic</strong> (alcohol-based) and applied with an airbrush (the goal is even coverage without strokes).</p>
<p>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.</p>
<p>Theoretically, dishwashing liquid is a surfactant, so if anything, it should <em>help</em> the paint (unless it has a specific reaction to the dye). But maybe someone has direct experience with this.</p>
<p>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...</p>

CC BY-SA 4.0
2021-08-25T17:40:52.043Z
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
CC BY-SA 4.0
2023-01-21T08:58:15.397Z
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
CC BY-SA 4.0
2023-05-26T14:43:26.510Z
It's also possible the detergent residue was on your finger and not on the surface.
Little Girl
CC BY-SA 4.0
2024-08-06T06:35:45.023Z
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
CC BY-SA 4.0
2024-08-06T08:44:00.687Z
@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
<painting><acrylic-paint><surface-preparation>
Detergent residue on the surface before painting - how bad is it?
Zeus
# Detergent residue on the surface before painting - how bad is it?

(Asked by: username_0 on 2021-08-25)

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...

**Tags:** painting, acrylic-paint, surface-preparation

> **Comments:**
> **username_1**: 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.  
> **username_2**: 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  
> **username_3**: It's also possible the detergent residue was on your finger and not on the surface.  
> **username_4**: 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).  
> **username_0**: @username_4, 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.  

---

## 3 answers

### Answer by username_5 on 2023-09-13. Score: 1

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.

> **Comments:**
> **username_0**: 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.  
> **username_4**: @username_0: 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 :)  
> **username_0**: @username_4, 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...  

---

### Answer by username_6 on 2023-01-20. Score: 0

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.

> **Comments:**
> **username_6**: And this answer is downvoted for what reason?  
> **username_4**: 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.  
> **username_7**: @username_4 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.  
> **username_0**: The Q specifies standard dishwashing liquid and alcohol-based acrylic, if you want to be specific. What is "standard acrylic"?  
> **username_6**: @username_0, from context, standard acrylic model paint. Such as GW or army painter, maybe? Standard as opposed to something like metallic or a speed paint?  

---

### Answer by username_8 on 2023-11-06. Score: 0

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.

> **Comments:**
> **username_7**: *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.  
> **username_4**: @username_7: 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.  
> **username_7**: @username_4 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.  

---

