Experimenting with caustic soda (sodium hydroxide) paint stripping

LewisCobb · Jan 4, 2015

Hi –
I have been doing a little experimenting with a home brew paint stripper made from a 10% mixture of sodium hydroxide (a.k.a. lye, caustic soda) and water. Basically 1 gallon of water and 1 pound of lye. To the 10% solution I added a bit of corn starch and mixed that in until the consistency was a gel so it would cling to vertical surfaces.

I swabbed some onto a vertical cast iron surface that has paint and primer residue and has been difficult to get off the rough cast surface. There were a few voids in the cast iron in the test patch that held paint as well. I covered the test patch with plastic food wrap to keep it from evaporating, and then left it overnight – probably 12-14 hours.

When I came back, the patch had a distinct reddish color to it, but scrubbing a little with some water and a brass detail brush, sure enough the paint and primer, even in the little casting voids, was all liquefied and came off very easily. A bit more water, scrub, then a squirt of acetone and wipe, and it was dry. However, there was a distinct yellowish tint to the area that was covered in the stripper. It looks like the aqueous solution of lye had started to rust the cast iron during the stripping process. The edges of the patch were well defined .

Other than this rusting, this paint stripping technique is the most effective I have ever used. It does not stink at all, cleans off easy enough, and liquefies the paint residue so that it flushes out of the voids. Sodium Hydroxide is of course dangerous if any gets on you, and you will most likely be blinded with a very small amount in your eyes, so one has to suit up properly.

Is anyone aware of refinement techniques to what I have done to minimize or prevent the rusting while it’s stripping? The temperature I was doing all this at was slightly below room temperature – nothing was heated. I have heard of tanks of this stuff being used to submerge items in, some heated some not so I am not sure if a warm/hot solution would be any different.

Thanks for any input.

Lewis

Limy Sami · Jan 4, 2015

Hot tank caustic dipping will shift paint fast, the hotter the better, rusting and corrosion is a problem., I think something the caustic soda promotes rusting, much aided by an oil free surface.

DO NOT use on aluminium parts.

If you want something a little less aggressive (and safer) use washing soda (sodium carbonate) - boiling machine parts in the stuff takes everything off, on a cool damp day you can watch machined CI parts rust as they cool.

Again DO NOT use on aluminium parts.

DMF_TomB · Jan 4, 2015

caustic

LewisCobb said:
Hi –
I have been doing a little experimenting with a home brew paint stripper made from a 10% mixture of sodium hydroxide (a.k.a. lye, caustic soda) and water. Basically 1 gallon of water and 1 pound of lye. To the 10% solution I added a bit of corn starch and mixed that in until the consistency was a gel so it would cling to vertical surfaces.

I swabbed some onto a vertical cast iron surface that has paint and primer residue and has been difficult to get off the rough cast surface. There were a few voids in the cast iron in the test patch that held paint as well. I covered the test patch with plastic food wrap to keep it from evaporating, and then left it overnight – probably 12-14 hours.

When I came back, the patch had a distinct reddish color to it, but scrubbing a little with some water and a brass detail brush, sure enough the paint and primer, even in the little casting voids, was all liquefied and came off very easily. A bit more water, scrub, then a squirt of acetone and wipe, and it was dry. However, there was a distinct yellowish tint to the area that was covered in the stripper. It looks like the aqueous solution of lye had started to rust the cast iron during the stripping process. The edges of the patch were well defined .

Other than this rusting, this paint stripping technique is the most effective I have ever used. It does not stink at all, cleans off easy enough, and liquefies the paint residue so that it flushes out of the voids. Sodium Hydroxide is of course dangerous if any gets on you, and you will most likely be blinded with a very small amount in your eyes, so one has to suit up properly.

Is anyone aware of refinement techniques to what I have done to minimize or prevent the rusting while it’s stripping? The temperature I was doing all this at was slightly below room temperature – nothing was heated. I have heard of tanks of this stuff being used to submerge items in, some heated some not so I am not sure if a warm/hot solution would be any different.

Thanks for any input.

Lewis

.
i used to use hot caustic 140F in a ultrasonic cleaner to clean printing press cylinders. i
used a overhead crane and the painted lifting hooks were stripped of paint on the first
dip into hot caustic
.
usually no rust forms if caustic is concentrated but rust will form if diluted too much.
.
caustic basically will dissolve skin and eyeballs too. i got some on my leg and even when
washed within 1 minute i was still minor burned. my legs in those spots gets red and dry
every winter. it heals but never totally healed. some damage is for rest of life.
.
it required diluting about 100 to 1 to get ph below 10 and we were not allowed to go
down the drain unless it when in a dilution tank was below ph10. obviously it will poison
a creek or river and wildlife unless below a certain ph. it would normally take a
swimming pool of water to dilute one gallon of concentrated caustic. you can neutralize
with acid but the reaction can be very intense and cause boiling explosion of caustic spraying
very far

sch · Jan 4, 2015

Your solution is a little more concentrated than most recipes, which suggest 5% NaOH, your solution is more like 12.5%. Your lye may have some amount of NaCl in it which causes
rust, where NaOH in low percentage solutions used for paint stripping that is not contaminated does not cause rust. Limy's suggestion is a good one as lye is extremely hazardous
on direct contact with skin and would likely blind you if accidentally splashed in an eye unless instantaneous very effective irrigation was done and I mean instantaneous*. A dilute
vinegar wash would be a good idea to neutralize residual lye, especially important with a gelled preparation. FWIW rinsing with a high pressure stream of water risks splashing
the rinsee with gelled lye. Washing soda (sodium carbonate) or TSP (trisodium phosphate) are both fairly strong alkalis on their own but somewhat less reactive on skin/eyes.

All of these make adequate paint strippers but there is a reason they are not sold as such, they are too dangerous for the uninformed to use safely. You need a tyvek full body
suit and full face protection for safe use, outdoors. *even then a corneal transplant will likely be needed to salvage vision.

challenger · Jan 4, 2015

I am admittedly pretty lazy when it comes to safety. I wouldn't say I'm reckless but I do sometimes spray paint without a good air mask and occasionally will not wear glasses (very rarely). I have to say I would NOT screw with this stuff. I am no chemist but caustic, in my mind, just means another way of doing the same damage that acid does.
Spooge tank. I know they don't come in handy with super large items but I made one from 1/2 a 55 gallon plastic drum and I have removed a lot of paint and rust etc on machines that looked ready for a dirt nap.
If the paint is stuck that well why do you need to remove it? Just wondering. I have a small vertical mill I'd like to get going on and it obviously won't fit in my spooge tank.
Thanks

LewisCobb · Jan 4, 2015

Thanks for the input guys.

I know all about the aluminum / brass / zinc etc. precautions - also know full well the dangers involved and am prepared to suit up for them too. I'm mainly just experimenting at the moment - there's a foot of snow on the ground outside, and I am jammed into the corner of my woodworking shop with this old lathe bed and headstock casting and wondering how to deal with the residue on it.

I'm so close to having this thing stripped to bare cast I am going to keep on going at this point. Besides, the stuff that's left is more residue and such, but very hard to wipe off with the roughness of the casting in some places. I started this experiment with NaOH after many hours of reading about it on here and other places (probably too many hours - I could have cleaned the casting with a toothpick in the time I have spent studying this stuff - lol).

I think the stuff I got (NaOH) is pure enough, but I can see if there was a bit of chloride in it, it would cause problems. The rust was not severe - more like a "hint" of rust. Still, I was thinking about scaling the whole thing up and making a tank big enough to submerge the entire casting in come springtime - but I am a bit leary of that now. There's no way I could get it out, flushed, and dried fast enough to prevent widespread rusting.

The casting is an awkward thing - the headstock and bed are all one piece - headstock does not come off. As a result, I'd need a tank about 4' x 16" x 16" to hold it and about 50 gallons of NaOH.

There are some places with rust on the casting that I'd like to remove but have not really come up with a decent way of doing that yet. Perhaps some of you can chime in on this one too. I had thought about some metal prep / phosphoric acid scrub but then read about some types of primers do not like the surface to be passivated with this stuff. At least the 2k primers I was looking into. That's another rabbit hole I have not gone down yet ....paint.

There's not a lot of progress happening at the moment given the time of year here, but I can do some "hand work" with a OV cartridge mask on in the garage. Anything serious is going to have to wait for spring and open doors etc.

Thanks again for all the input. Appreciate it.

Lewis

PS - Is it possible to scrub a surface with phosphoric acid metal prep and then flush it with water/acetone whatever, and not have it leave a residue that might not be compatible with some primers? i.e. a derusted and bare cast iron surface?

LewisCobb · Jan 4, 2015

challenger said:
I am admittedly pretty lazy when it comes to safety. I wouldn't say I'm reckless but I do sometimes spray paint without a good air mask and occasionally will not wear glasses (very rarely). I have to say I would NOT screw with this stuff. I am no chemist but caustic, in my mind, just means another way of doing the same damage that acid does.
Spooge tank. I know they don't come in handy with super large items but I made one from 1/2 a 55 gallon plastic drum and I have removed a lot of paint and rust etc on machines that looked ready for a dirt nap.
If the paint is stuck that well why do you need to remove it? Just wondering. I have a small vertical mill I'd like to get going on and it obviously won't fit in my spooge tank.
Thanks

There's a term I never came across - spooge.....

I actually have a spare plastic 55 gallon drum I got last summer and figured I might be able to use it for a dip tank in the future. It was free so I'm not really out anything if it does not pan out.

9100 · Jan 4, 2015

There are commercial strippers that leave a phosphate surface that is resistant to rusting and is a good one for paint to bond to. All this research has been long done and a little research should uncover it instead of reinventing the process. When I had access to it, I used to hang motorcycle parts in a vapor degreaser to remove every bit of oil and degrade the paint, then put them in an ultrasonic cleaner filled with Oakite Ruststripper. The paint came flying off.

Bill

sch · Jan 4, 2015

challenger;2453062. I have to say I would NOT screw with this stuff. I am no chemist but caustic said:
Minor point on alkali versus acid and human tissue: strong mineral acids tend to coagulate proteins which then form a barrier to further tissue spread. Alkali on the other hand liquefies tissue and
saponifies fat so it just keeps on penetrating. Doctors would MUCH rather deal with an acid than alkali exposure. Hydrofluoric is an exception as it too keeps on spreading and the fluoride ion
toxicity is pretty high, it mainly being stopped by forming CaF, which is insoluble, but there is not enough Ca++ around unless you inject some. The acid/alkali chemistry is one thing, toxicity is
another and separate consideration.

Limy Sami · Jan 5, 2015

On the subject of using hot washing soda (sodium carbonate) for parts cleaning.

IME there's nothing like it for dried oil and grease, and especially dried coolant, taking all the chips and crap with it from parts like lathe aprons and mill saddles.

I rinse off with cold water from a hose, the large castings and lumps not cooling much so they dry themselves.

JoeE. · Jan 5, 2015

The railroad shop here in town used to have a big tank that they dipped stuff in to clean. I think I recall it being called the "oakite tank".

challenger · Jan 5, 2015

What? Never heard of "spooge"? That's a subject for an entirely different thread. Actually a different forum

I've used ospho etc and it seems to neutralize rust but leaves a powdery residue that has to be tended to so I usually end up chasing my tail with the process.
I know this will get some flack but if it was just some flash or thin surface rust I would etch prime, seal and paint the part. I wouldn't do this to a car but I've done it to machine parts that would rust in a matter of minutes. I feel rust isn't the monster everyone makes it appear to be. Some people think if there is a speck of rust left under paint it will eat it's way through to the other side. I feel of it is encapsulated it won't be a problem. I'm not talking about the thick flaking rust of course.
I started using Sherwin-Williams Sher-wood enamel and a hardener over primed metal. I spray it and it builds up nice and slick so wiping down the parts is easy. It is also the hardest paint I've used. For some small parts in use a powder coated close by. The danger there is any oil will bake out and casting have plenty of that hidden inside.
Good luck.

BobRenz · Jan 5, 2015

MANY moons ago, we had a caustic strip tank at the railroad shops. We used a commercial Oakite product that was mostly sodium hydroxide, with some other additives to boot performance. We used a hot tank - the kind with a flame tube running through the tank, and we ran it close to boiling. It would peel darn near anything off iron/steel, but anything that was aluminum would dissolve in seconds. Be VERY careful with caustics - we were warned that caustic in your eyes could wipe out your vision in a heartbeat. We used shoulder length rubber gloves, rubber aprons, goggles, and face shields - even on hot days, everyone obeyed these safety rules 100%.

When the tank wasn't in use, it was covered and padlocked.

LewisCobb · Jan 5, 2015

These Oakite tanks sound like something straight out of a James Bond movie....

I realize all this stripping/derusting/painting business seems to have been beaten to death on the machine forums, and after reading just about all the threads I found, I realized why it comes up over and over - everyone has to deal with their own set of variables and restrictions. I got pretty discouraged when I saw some of the setups people have on here for refinishing things with full spray booths, blasting booths, forklifts to move equipment around - lol. Still, I am soldiering on very carefully in a corner of my woodworking shop....

Sounds like I'll stay away from that Ospho - I'm chasing my tail on too many things at the moment anyway.

One thing that is working - albeit brutally slow - is an acetone spray bottle and a little brass brush :eek:

I do have a OV cartridge mask to wear while I do that. It's really just a spot treatment though.

I was quite surprised in that many parts of the machine are literally shiny smooth under the paint - and I mean smooth. I would be worried about paint flaking right off it. The lads that built these machines back in the 60s must have put countless hours into refining the castings before painting. I suspect that an etch primer will be in the cards when the painting starts.

Now then - a bit off topic from stripping paint - has anyone here used Interlux brushable 2 part polyurethanes on their machine? They make a 1 part poly as well - Brightside it's called. Designed for boats, it seems like it might be an ideal coating for someone that is unable to spray, and even in the case of the 1 part Brightside, it sounds like it might be more durable than the modern reformulated alkyd enamels that never seem to harden.

Thanks again for the input and sorry for putting up yet another finishing thread

dgfoster · Jan 5, 2015

LewisCobb said:
These Oakite tanks sound like something straight out of a James Bond movie....
<>

Thanks again for the input and sorry for putting up yet another finishing thread

Could be the last movie you will ever see. :bawling:

The problem with the caustic soda and melting eyeballs has been stated but is very real. We live in a world filled with seemingly useless (CYA) cautions on every product we buy so that we tend to disregard all cautions. No way I would consider using caustic soda for stripping unless it was in a full-on engineered setup.

Denis

APD · Jan 5, 2015

I was doing some electrolytic rust removal, using washing soda, and found that it removed both rust and paint. If you have a car battery charger and a tank big enough for your parts, you might try this less toxic option.

Newman109 · Jan 5, 2015

When I was but a mere child back in Chicago, I had a desire to ride motorcycles. I had little money but lots of energy. Motorcycles were cheap then and my first project was removing the paint from the tanks and fenders of my new to me $25.00 1942 Harley 45.

We had a large soapstone sink with two compartments in the basement of our old house. I would fill that up with very hot water and then dump in about eight or ten cans of Buckeye Lye (10 cents a can so why not?). I have no idea what concentration this was, but it was pretty hefty. The parts were then lowered into this mixture.

Then I would wait until the water cooled down and remove the parts using the wires that I had attached. I would rinse them thoroughly in the other side of the sink. They would be perfectly clean, bare metal with no signs of paint.

The bottom line is that it's pretty hard to beat lye for removing paint. As mentioned above, just don't use it on aluminum.

On the downside, you would have to prime the parts immediately after drying them out or they would start to rust.

On the good side, we had the cleanest sewer pipes in the neighborhood.

adama · Jan 5, 2015

Can't remember when but there was a bit on the BBC about dissolving bodies in caustic soda as opposed to the environmental damage of cremation. Seam to recall it was at a similar speed process. That alone should justify the safety comments :-O

Garwood · Jan 5, 2015

I've used caustic soda for years in washers large and small and ultrasonic tanks. I've got it in my eyes numerous times and never heard of what you guys are hollering about. It sure burns, but you flush it for a minute and your fine. I'd much rather get caustic soda in my eyes than gasoline.

About the worst thing that happens with caustic soda is that your skin will get slimy (start to disolve) if you're not wearing gloves and are in contact with a lot of it.

I've always used it hot. The stuff doesn't seam to work at all cold. Hot it'll strip anything real fast.

crossthread · Jan 5, 2015

There is a guy down the street that refinishes furniture. He seems to do a lot of wicker which is a real pain. He uses an old bathtub to soak the parts in using lye and water. What may be unique about what he does is that he pulls the stuff out and while it is still wet he uses a pressure washer to remove the paint, lye and what have you. It does a great job with little to no effort and when it dries it is ready to spray paint again. I don't see why a pressure washer wouldn't work on what you are stripping to get rid of anything that has been softened up.

Experimenting with caustic soda (sodium hydroxide) paint stripping

Aluminum

Diamond

Diamond

Hot Rolled

Stainless

Aluminum

Aluminum

Diamond

Hot Rolled

Diamond

Titanium

Stainless

Stainless

Aluminum

Diamond

Stainless

Diamond

Diamond

Diamond

Titanium

Similar threads