How to wash milk painted surfaces?

About two and a half years ago, I made "Slow Down" signs for our road and painted them with milk paint (more about using milk paint for fine art). Our road is a gravel road, and we get a lot of rain here. The signs are in serious need of a clean and repaint.

I'm wondering what best to clean them with. I don't want to remove the paint--definitely not anything acidic, as the paint is alkaline. The Real Milk Paint just says to "use gentle cleaners, avoiding harsh chemicals"....but doesn't give any recommendations for what gentle cleaners to use. Of course, this is probably talking about milk paint furniture, which gets a lot less grime than a sign by a gravel road.

I'm thinking of using a damp rag with either Murphy oil soap, or Dr. Bronner's soap. Not sure which would be bettter.

I'm starting with the worst of the two signs. It's the one on the right of this image:

This is how the signs originally looked

This particular sign is in pretty sad shape, because it had fallen over last year and was covered in bramble and a lot of paint had eroded off of it. I had repainted it last year. I'm pretty sure I used my own homemade milk paint for the white, rather than using the expansive just-add-water Real Milk Paint.

I'm pretty sure the brown is mildew damage, as the dirt didn't wash off like on my other sign that is closer to the road. If nothing else, all this cleaning is good prepping for me to repaint it!

The top layer of white paint (I'm pretty sure I made it myself with milk, lime, and calcium) is pealing off. The underlayer of Real Milk Paint is holding up fine.

So, I waited 30 minutes and went out and scrubbed at it some more. It did manage to remove more of the mildew staining, but not all. More of the homemade milk paint scraped off, but not much of the Real Milk Paint.

I'll definitely be repainting.

I started repainting yesterday evening (yestereve?), and grabbed a bunch of my Real Milk Paint containers. I thought I'd grabbed a container of white, only to realize it was...

[img]https://m.media-amazon.com/images/I/7175OIZKdoL._AC_SL1500_.jpg caption="real Milk Paint's Outdoor Additive"

And, when I had painted my signs the first time, I had added this to the paint. When I repainted this sign the second time, I'm pretty sure I did not use it. That might explain why the outer coat had so much mildew, and the first layer of paint did not.

So, I added it to my paint this time, too.

I'm going to let the paint cure for a few days before adding tung oil to weather proof it.

One thing my husband was impressed at was how resistant the paint was to all my scrubbing and blasting with the jet setting on my house. Most of the paint stayed on the wood and only a little flaked off (most of it being the layer of paint I'd done without the Outdoor Addative).

It looks amazing.   Even better than a simple clean.