DIY Caustic Paint Strippers

My tumble down house never seems to not need some work doing on it. At the moment that means I’m restoring the sash windows at the front of the house. They are about 200 years old and probably haven’t been touched in 30 years so they are in a bit of a state. On the upside they were well made back in the day and while I’ve sure a professional would just scrap the sashes I intend to keep as much of the original woodwork as I can. That means I’ve got to get 200 years of paint off first. While I haven’t tested it I think it’s safe to assume that at least one of the 342 lays of paint contains lead so I’ll be treating it as such.

In the past I’ve hauled timber outside, donned a mask and had at it with a heat gun. As long as you are careful you can strip a lot of paint very quickly with minimal risk. The windows, however, pose a challenge, I can’t take the sash boxes outside to strip them. I’ve tried solvent based paint strippers before and they were a dead loss on lead paint. They are awful to work with and they barely touch the paint. After some research I came across a product called Kling Strip which promised to eat through old paint like it wasn’t there.

Without much expectation of it actually working I bought some Kling Strip and gave it a go. What do you know, it works. It comes as a paste like substance that you put on the woodwork and then cover with plastic sheet while it does it’s thing. Given a thick enough layer and some time it’ll eat through some seriously thick paint.

That’s where the good points end, though. It’s quite a lot of faffing about compared to hot air stripping and it’s fairly expensive stuff. I bought a 5 litre tub for £32 which is about enough to do one of my smaller windows. It’s not going to break the bank but I’d like something cheaper. So…

I looked into the chemistry of it and the active ingredient is just sodium hydroxide (caustic soda, lye) which is one of the cheapest chemicals you can buy. I looked around to see if anyone else had thought about this and I found a couple of recipes online. The top result suggest using corn flour to thicken a sodium hydroxide solution. I tried this and it was a complete disaster. The sodium hydroxide denatured the corn flour in an instant and it turned into a gloopy mess that wouldn’t spread.

I looked at Kling Strip again than it hit me what they were using as the filler material, clay. I don’t think that clay is the only other ingredient but I’m essentially sure that’s the bulk of it. There seems to be something else making it a little more spreadable but I’ve not worked out what yet.

Recipe for DIY Paint Stripper

  • 1000ml water
  • 1000g bentonite clay powered
  • 80g sodium hydroxide

Mix the ingredients together taking care to not splash the mixture all over the place as it’s quite basic. Sodium hydroxide releases a fair bit of heat when it’s hydrated so you’ll notice the mixture becomes warm. If you add much more sodium hydroxide than this you’ll need to take some care not to boil the mixture when you add the water, this should be fine though.

This will make a 2M sodium hydroxide paste which seems to be more aggressive than Kling Strip. Adding a little more clay, say 10%, seems to make it a little more sticky which will aid spreading. Remember that 1000ml of water weighs 1000g so you only need a scale.

This mixture is too runny for overhead work so for that use Kling Strip as it’s tacky. If you can get the work piece down on the level though my mixture will get the job done faster and cheaper.

Cost wise, I bought 1kg of sodium hydroxide for £7, that’ll make at least 10 litres of stripper. The clay is more expensive. I initially bought 1kg for £6 as a test. That would make my mixture about £6.70/litre vs Kling Strip at £6.40/litre bought as a 5 litre tub. Looking around I can 25kg of clay for £40. That would require about 2kg of sodium hydroxide (£14) for a total cost of £54 for 25 litres making a final cost of £2.16/litre. A similar quantity of Kling Strip would be around £140.

To be honest, even if my was a little more expensive I’d still use it as it’s much faster. Mine can be ready to strip in an hour whereas Kling Strip usually needs to be left at least over night and I often leave it a couple of days.