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 65C, 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, a unexpected but great benefit. I removed the worst remains of the tube glue with a JLC razor blade used as a scraper.


More experiments

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 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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