The sprinkler water has a high iron content. After a time the siding is very orange.
Did you give CLR a whirl? That might take care of your problem.
The proper chemical being muratic acid.. 1 part acid to 2 parts water should do it. You can always add a little more acid. When it turns the rust green its working. Let it dwell for a couple minutes and rinse thoroughly
I agree with Becky, I would try CLR. You could try the organic material method here: https://www.jameshardie.com/blog/tips-for-cleaning-your-siding?loc=refresh
Check the manufacturer's website. I'm sure this has happened before. Stay safe!
Hi Jean, I hope this helps you out,
We have Hardie board siding and haven't experienced that. Yes I would try CLR as well. Also, when you go to repaint, use a GOOD Quality paint. We used Sherwin Williams Duration and although it was much more expensive than other outdoor paints, it hasn't faded AT ALL!!! It has been at least 6-8 years ago that we painted. Something to think about for the next time.
RustOleum makes a rust remover - https://www.lowes.com/pd/Rust-Oleum-32-OZ-Spray-Gel-Rust-Dissolver/1002035750?cm_mmc=shp-_-c-_-prd-_-pnt-_-google-_-lia-_-133-_-thinnersandsolvents-_-1002035750-_-0&store_code=580&placeholder=null&gclid=EAIaIQobChMIjp61pdSu6gIVDr7ACh0JJwlmEAQYAiABEgKhRfD_BwE&gclsrc=aw.ds
I would use that and then wet & forget outdoor to keep it clean.
Here's a post that might help - http://www.propowerwash.com/board/upload/showthread.php?32735-Rust-On-Hardie-Siding