What do you recommend to wash down walls before painting?

I don't always wash my walls before I paint. Sometimes I just vacuum them off. It depends on what walls I am painting. I would just use some Mr. Clean and warm water in a bucket. You can use TSP but wear gloves as it's hard on your hands. This article tells you why you may not have to wash your walls before painting.

https://www.thespruce.com/washing-interior-walls-before-painting-1821152

TSP, trisodium phosphate

Sandy I always do, with sugar soap and hot water, apologies this may be a British product. Cleaning it down gets rid of any dust, sticky marks, dirt, cigarette smoke etc. Failing to prepare properly often gives a poor finish and there is nothing worse than spending a lot of money on paint and ending up with a really poor job.

How to Prep Walls for Painting - Sherwin-Williams - YouTube

https://www.youtube.com/watch?v=hryKCioCRXQ

TSP

You only ned to wash your walls if they are dirty from cigarettes, grease, or some other grime. If your walls are clean and you only want to change the color, it isn't necessary to wash them. Of course dust away any spiderwebs, etc. before priming.

TSP is excellent to scrub any surface before painting. It's an easy mix powder you can find at Home Depot and it's inexpensive, just mix with water and wipe.

I almost never wash my walls first. Except in the kitchen. Then I use water and vinegar. If you use any kind of soap (or even vinegar), make sure you then wash it again with plain water.