Ultrasonic NaOH paint stripping



Almost by accident I discovered a perfect way to strip off paint from plastic models. In the past I always used sodium hydroxide (NaOH) oven cleaner from a spray can, that worked well but slow, and often required multiple applications and considerably scrubbing. But when I combined it with an ultrasonic cleaner I found the perfect method: all paint and decals were removed in 10-20 minutes without any paint traces left. The technique described here works on enamel paint, I haven't tried other paints yet.


Shown here are the two main ingredients: sodium hydroxide (NaOH, lye and caustic soda in the US) and an ultrasonic cleaner. I used a drain opener that is a solution of approximately 10% NaOH in water and probably nothing else. I diluted it further down by adding water, to maybe 5%.



Warning

Be careful handling NaOH! Your eyes are at risk, use safety glasses. Gloves are recommended too, otherwise your skin will start to feel greasy and slippery, because it's slowly dissolving.

 
  From what I read, stainless steel is not attacked by NaOH below 65°C, but still I put the NaOH solution a separate plastic container, suspended in the metal wire basket that comes with the ultrasonic cleaner. There are no liquids in the photo, I did not want to get my photo set-up wet!
I think the wire basket can be left out, with the plastic container with NaOH solution directly in the water of the ultrasonic cleaner. This is the setup that I used ever after.  
  This is the model I wanted to strip: a 35 year old Tamiya 1/20 McLaren M23. I used a Humbrol enamel spray can for the white, and brush-painted everything else with Humbrol enamels. I remember overpainting the large red-orange decal areas with red paint some time later.
All paint and decals were removed in 10-20 minutes. The parts were perfectly clean without any further effort, no scrubbing or anything.  
  Interestingly all glue joints also broke down during the paint stripping, an unexpected but great benefit. I removed the worst remains of the tube glue with a JLC razor blade used as a scraper.


More experiments

I stripped many more old model parts that were painted with almost exclusively Humbrol enamels. Most of the stripping went fast and gave perfectly clean parts, like this cabin of 35-year old Ertl COF-4070B Transtar II. It had at least four different paints on it, with some paint drips, but everything came off perfectly.  
  But I also had some parts that would not clean 100%, like this 40 year old Tamiya 1/20 Tyrrell six-wheeler. I had built it without painting, but later overpainted with matt black, followed by yellow flames. Interestingly, the black beneath the yellow would not come off. And contrary to the M23 above, the few glue joints did not brake down.

It is my impression that parts will try to find the areas of least excitation if left to their own device. They then clean less than in areas of high excitation.

One of the unknowns is the strength of the NaOH solution, since I buy it as drain cleaner with an unknown concentration. Therefore a next step could be to buy NaOH in dry form, and be able to decide on the concentration myself.

Another unknown is when to replace the NaOH solution. Does the stripped paint deminish the stripping strength of the solution? My chemistry knowledge is severely lacking here.

A question that applies to all paint strippers is how damaging the stripping is to the polystyrene material. Chemical compatibility tables and charts for polystyrene show a reasonably good resistance for various NaOH concentrations and temperatures. But this information is too generic and undetailed to draw conclusions on.

The disbonding of all glue joints, as experienced with the McLaren M23 model shown above, did not occur during most other stripping processes. I cannot explain this difference.

For acrylics my first guess would be to use household ammonia. We tried it on a tank model painted with Tamiya paint, but nothing happened. Alternatives are cleaning alcohol, or isopropyl alcohol (IPA).

I had a roughly 25-year old tin of Humbrol that I wanted to use. I stirred it a bit, to mix in the thick drab at the bottom, then got the idea of putting it in the ultrasonic cleaner. I put it in a plastic bag and put it in for 10 minutes. There still was a bit of thick paint at the bottom, but a bit of shaking solved that. By then the paint was really well mixed, and had a really low viscosity. Amazing! More experiments are called for.





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