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Are there any recommended ways to remove acrylic model paint from a PET or PLA model kit that has not been varnished.

For example, to allow old kits to be stripped down and repainted once your skill level has improved.

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  • Just to clarify, are you talking about model car/plane/Gundam-style kits, or wargaming kits? (I'm not sure what type of plastic each uses, and googling only gets me listings for Gunpla kits.)
    – Allison C
    Jan 7, 2022 at 14:47
  • Various kits, Warhammer, for example uses polystyrene. Jan 7, 2022 at 17:04
  • Airfix (planes etc.) certainly used to be PS too
    – Chris H
    Jan 11, 2022 at 11:58

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There are a number of products commonly recommended for stripping miniatures by mini painters of various experience levels; as with any solvent, each of these should be tested on a spare piece of the same material or a portion of the model that won't be visible to ensure they won't affect the underlying plastics.

The most commonly recommended solvent is a cleaner called Simple Green (in the US) or Biostrip (in the UK). From Age of Miniatures, these are described as "very gentle on the miniature, because it is water based. The chemical is not as aggressive – just stick your miniature into it, get it out an hour later and finally brush all the paint off." These are the ones I've seen most frequently referenced on various mini painting sites and forums.

Another that should be safe on most plastics is isopropyl alcohol. Higher concentrations will work better; I've had good experience removing paints and sealants from resin pieces using 91% isopropyl, and have heard from others that the more commonly available (lower) concentrations are considerably less effective. In the US, at least, 91% isopropyl is readily available in most areas that sell first aid supplies.

Most other solvents I've seen referenced carry a risk of dissolving the underlying model and/or carrying health risks to the user.

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I'll assume you did a good job of preparing the surface so the paint is well-bonded, and the old paint has long since fully hardened. So techniques for removing recently-dried paint aren't likely to have much effect. You've also probably weighed just painting over the old paint and that wouldn't be satisfactory. To strip the paint, you'll need something that attacks dried acrylic paint but not PET or PLA.

If we stick with common, readily available materials, here are five that work pretty well to remove dried acrylic paint from non-porous surfaces:

  • Lacquer thinner. Some of the common ingredients in lacquer thinner are hazardous, and some lacquer thinners have been reformulated with greener, safer ingredients. Unless you have some chemistry knowledge, it can be tough to know how a particular lacquer thinner product will behave with these materials. At least PLA is attacked by some of the ingredients commonly found in lacquer thinner. So I would probably just avoid this option.

  • Acetone. PLA is attacked by acetone. PET would tolerate the amount of exposure needed for cleaning, especially at room temperature (it is rated as being poor for long-term exposure). If you can easily and reliably identify which pieces are PET, acetone would probably be the fastest cleaner, but it's the least safe. There are other listed solvents that carry little or no risk for the model and are effective, so I would eliminate acetone, also.

  • Household (sudsing) ammonia. This doesn't so much dissolve acrylic paint as break down the film. The fumes require good ventilation and you need to be careful what tools you use with it (e.g., if you brush it on, it isn't friendly to any wood, brass, or aluminum that may be brush components). PLA is supposedly safe at room temperature, but heated ammonia can encourage it to breakdown. PET is rated poor with ammonia in concentrations greater than double the typical household product. Bottom line: you could probably get away without damage from cleaning either plastic with it, but it wouldn't be my first choice.

  • Denatured alcohol. This is a mix of ethanol and methanol with no water dilution as with rubbing alcohol. You can find it in the paint department. It is a pretty good solvent for dried acrylic paint. PET is impervious, and PLA nearly so to the alcohol (a test of alcohol in a PLA bottle resulted in some white discoloration after 16 weeks).

    However, some of the substances that can be added to denature it attack PLA. The denaturant is at a low concentration, but your model is likely to be soaking in it for awhile. If you are going to use this solvent on PLA, you might want to soak a scrap piece in it for several hours to see if it is affected by that brand. Also be aware that methanol is poisonous and pretty flammable, and burns with a flame that is nearly invisible in sunlight so it could take longer to realize there's a fire if it ignites.

  • Isopropyl alcohol (the higher the concentration the better, preferably 91% or higher; you may need to shop around pharmacies to find it at those concentrations, or order it online). This isn't quite as effective as denatured alcohol, but it will work, and it is safe for both PET and PLA.

My choice would be either type of alcohol (subject to the precautions mentioned).

The general method is to soak the item in the alcohol and check it periodically (start with every 15 minutes or so, and judge by progress). If you left it in the alcohol long enough, you could probably wipe the paint off. But the paint will soften and you can speed the process by removing paint as it does. Use a plastic scraper to push off softened paint, and a soft toothbrush or something similar to scrub paint out of crevices.

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    I would add that the soaking is best done in a container with an airtight lid. And I probably wouldn't use that same container to store foods anymore afterwards.
    – Elmy
    Jan 7, 2022 at 5:37
  • Denatured alcohol: Ethanol is also flammable (though of course not nearly as toxic as methanol, for which inhalation is an issue). In some places denatured alcohol no longer uses methanol but isopropanol - less toxic, a more visible flame, and probably a tiny bit safer for plastics than the old formulation assuming that the other additives aren't an issue. Isopropanol from pharmacies is often of the rubbing alcohol type, and I wouldn't be confident of the additives in that either (certain oils that give it the distinctive smell)
    – Chris H
    Jan 7, 2022 at 12:33
  • I have gallons of IPA, kept in a chemical cabinet. Jan 7, 2022 at 16:49

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