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Having seen some impressive results from other people using caustic soda as an RC paint stripper I've been keen to try it myself ... and having also seen some case where it's been no use whatsoever, I wanted to try a few permutations of paint & base materials...

I bought a 500g pack of caustic soda from Homebase ... looking at the instructions, 3 levels of solution strength are indicated:
• General sanitising/cleaning: 5% w/v (weight into volume, e.g. 250g caustic soda into 5L water)
• Drain cleaning: 10% w/v (e.g. 100g caustic soda into 1L water)
• Drain unblocking: 20% w/v

This stuff will cause burns, so do read the safety warnings - rubber gloves and eye protection are required. I used vinyl gloves without problems, I don't know about nitrile or latex, I didn't try them.


Experiment #1: Factory 1K Paint on Styrene

I started with the strongest recommended solution of 20%, putting 40g of the caustic soda into 200ml water - the solution quickly got warm (around 50 Celcius) - which explains why you should never pour water into soda crystals

First in the bath was a set of wheels from a Spital Trial 12, complete with original aluminium look paint & some vintage dirt ... the dirt fizzled off quickly, but even after 24 hours with regular agitation & occasional scrubbing, there was no evidence of the paint coming off. After washing the wheels off, the paint appears to have gone a little darker, lost some shine & had softened a little - a very small percentage would come off with scraping.

Overall: FAIL - no paint stripping ability observed. On the plus side, the plastic has survived ok.

Maybe a stronger solution would help?









Experiment #2: Tamiya X/XF Acrylic Brush Paints on Styrene + Resin

Next up, a Tamiya driver bust, Wild Willy NOS bottle & several repro resin heads, all of which I hadn't painted that well. I started with the 20% solution ... immediately, the "Flat Aluminum" (XF-16) started to fizz, and was gone within minutes. None of the gloss paints, clears, or the permanent marker showed any reaction; however the "flesh" paint did appear to be getting lighter.

After 6 hours (including regular agitation), I scrubbed everything with a toothbrush: approx. 1/2 of the "flesh" paint and the blue on the NOS bottle came off, 10% of the satin black, matt white & gloss yellow, but none of the "clear" green, orange and "smoke".

At this point I doubled the solution strength (i.e. 40% w/v - 80g soda into 200ml water) ... this had no immediate effect... nor was any greater paint stripping ability seen after a further 6 hours.

Overall: FAIL - paint stripping ability varies between rapid & total ("aluminum" paint) and none (Clear Green, "Smoke", etc).








Agitating with a brush does seem to help, but given the seriousness of the injuries you can do yourself with caustic soda, flinging droplets of it around is NOT to be recommended. If only there was a "brushless" way of doing it ...


Well, there is...

Experiment #3: Pre-treat as per #2 (Tamiya X/XF Acrylic Brush Paints on Styrene + Resin), Followed by Ultrasonic Bath

I'm a big fan of the ultrasonic bath for cleaning RC parts - it's not that it's that quicker or better than scrubbing with Fairy liquid & a toothbrush - but other than disposal of dirty water, it does mean you don't have to use the sink, and the chances of losing small bits are greatly reduced. The big plus for me is that you don't have to devote your attention to it - you can do something else while the bits are buzzing.

Anyway, my cunning plan was that if the caustic soda had softened and/or loosened the paint, then ultrasonic treatment might get the paint off better (and much safer) than scrubbing... and it really did. Within seconds paint was dissolving away, and within minutes all the matt/satin paints had almost entirely gone, as had the gloss yellow & blue. The clear orange, clear green and "smoke" (along with the underlying "Aluminum" didn't do quite so well, but I'm hopeful a repeat treatment would do more.

Overall: Win!

Experiment #4: Tamiya X/XF Acrylic Brush Paints on Styrene + Resin - Ultrasonic Bath (no pre-treatment)

This one was important to demonstrate that it was the caustic soda doing the business, not just the ultrasonic cleaning ... as expected, other than a tiny bit of paint loss due to "fretting" on the ultrasonic bath (and each other) the heads I tried emerged unscathed.








Experiment #5: Automotive 1k paint & Polycarbonate Paint on Polycarbonate, Followed by Ultrasonic Bath

While I can't guarantee the types of paint used on the polycarbonate here, I'm very confident in saying that the white was a 1K automotive aerosol (it wasn't stuck, and had damaged the material) and proper polycarbonate paint (stuck very well).

After a 20 minutes soak in 40% w/v caustic soda solution, the paint was becoming soft and gloopy ... a single, 8 minute cycle in the ultrasonic tank fetched 95% of the paint off, I'm confident the 5% left over was caused mainly by not using a container quite big enough for the driver figure, and would come off with a repeat treatment.

Note the pic of ultrasonic cleaning in progress - the last time I saw anything like that, it was being poured by Aunt Beru ...

Overall, a big win: very effective at getting paint off, with no damage being caused to the polycarbonate vacform.












Further experiments I need to do are to soak & ultrasonic the Spital wheels, and repeat the cycles where not all the paint came off. I'm almost (but not quite) tempted to try caustic soda solution directly in the ultrasonic tank.





Limitations ... obviously there are some - I haven't covered every paint type, as I didn't have anything else that needed stripping ATM. It's also very difficult to identify was paint someone else used 2 or 3 decades ago. Another important caveat is that previous paint stripping threads on TamiyaBase have pointed to there being many different types of polycarbonate/lexan that have reacted differently.

Overall, using caustic soda is higher risk, but less stinky than using De-Solv-It Graffiti Remover, doesn't cause any surface melt on styrene, and doesn't appear to cause damage to polycarbonate materials.

It's also much faster than brake fluid, and again, doesn't damage polycarbonate.

On the minus side, scrubbing seems to have little effect, making an ultrasonic tank essential. And, unless you can afford a tank big enough (or make your own), stripping paint off a whole body is going to be out of the question

Please be very careful in using Caustic Soda on Polycarbonate.

Caustic Soda will destroy "vintage polycarbonate" making it cloudy and brittle. I have had my own very expensive experience with this...
It seem to work nice on some polycarbonate (lexan), and it seems to be more recent materials that can take it. But in general, be very, very careful.

-Lars

Lars I had the same experience than you, I wanted to unpaint a old ford 34 hot rod body with oven clean bomb with caustic soda and after 1 week I fund some exploded lexan parts...

Hmm...
Great thread/experiments Jonny, nice factual approach & dedication!

My 3p's worth -
Clear paint/laquer is always a pig to remove no matter what method you use. Stands to reason really, as it IS supposed to be a protective coating for the underlying paint.

Yes caustic burns your skin which is unpleasant (Takes several minutes before you notice the stinging though) but the main concern should be that it WILL blind you if you get it in your eyes. In summary, goggles are more important than gloves, & don't splash the stuff about, which includes using a brush/scrubbing

Although I'm not 100% sure, I think my Mu had been stripped with caustic or oven cleaner, & although the ABS was clean, it's incredibly dry & brittle & can't be glued with styrene cement or CA. Whether or not this was an immediate effect or appeared after some time, remains to be seen.

I used to use caustic soda for cleaning out 2-stroke exhausts. 1 whole 500g bottle in a plastic 3-litre bottle of water, immersed in a bucket of cold water to keep it cool whilst mixing. This mix dissolves oily carbon deposits, will discolour chrome plate & take high temperature automotive spray off the mild steel sections of pipe.
This mix gets hot enough to properly melt coke bottles (Bottoms fall out), hence the cold water bath. Probably a bit warm for toy car bits so cooling prior to use would be advised!

I was working on a long post refuting each of the nay-saying posts in depth, but I realised it undermined my parallel argument about the essential pointlessness of debating sincerly held but completely cuckoo beliefs



I think I can distill it down to 3 points:

1: 16.6recurring% w/v < 20% and 40%

2: Oven cleaner <> caustic soda

3: 30 year old polycarbonate Spital driver/cockpit = vintage polycarbonate

Resin heads were a lot better after a 2nd cycle, but one took a 3rd... polycarbonate driver is now completely clear of paint.

I meant to only leave it in the caustic soda for 10 minutes, but left it in for 90 - if I'm wrong and it does go cloudy/brittle, I won't know if it's from being exposed full stop, or being exposed too long.



edit - forgot to mention the wheels ... no improvement at all

Do you leave the solution exposed to air or do you put a lid over it?
the solution will lose strength if exposed to CO2 from the air.

other thing you might like to test is to raise the temp of the bath (makes the paint softer, but not too high as to melt the plastic )

Jonny wrote:

I was working on a long post refuting each of the nay-saying posts in depth, but I realised it undermined my parallel argument about the essential pointlessness of debating sincerly held but completely cuckoo beliefs

I think I can distill it down to 3 points:

1: 16.6recurring% w/v < 20% and 40%

2: Oven cleaner <> caustic soda

3: 30 year old polycarbonate Spital driver/cockpit = vintage polycarbonate

JR,
It's not a belief, it's based on experience, just like you.
I have damaged a lot of shells (slow learner...) like the Celica Gr.B, a Opel Ascona and several more.

I have also done successful Caustic Soda Polycarbonate treatments, but only on later models, like from the late 80's/early 90's.

Polycarbonate comes in variations from different makers (Lexan, Makrolon, Macroclear and lots of others (thanks Wikipedia...)). Which one Tamiya used at certain times is not known.

I believe the only safe thing to say about Caustic Soda on "Clear plastic shells" is that it's a gamble. Especially if the shell is old.

-Lars

Found this on the net:
Chemical Resistance of Tuffak, Lexan and Makrolon Polycarbonate

-Lars