How do I clean grime off my patio siding?

Try Clorox cloeanup and a scrub brush then wipe off

Clean the siding with a pressure washer ever so often.

Can you power wash it?

Should be able to be power washed off. I have had the same problem, do you have trees over the area? There is a fungus that can drip off of the trees onto your surfaces. If that is the case, the trees need to be sprayed to prevent the black from coming down on your patio.

Hi Sandy. I'm Cindy. Dawn dish soap works great on grease and grime in the kitchen. I would fill a bucket with warm water and a couple squirts of blue Dawn dish soap. It's a lot cheaper than investing in a power washer, Finding the source is important to stop it from happening again. Good luck Sandy.

Tried this also 409 and goof off none of these worked

White cleaning vinegar with blue Dawn applied through a pressure washer or spray on, scrub if needed, and rinse off with a high powered nozzle.

If interior, it just might be black mold. Cleaning vinegar will kill the mold spores. Chances are the room needs better ventilation and installing a dehumidifier would be something to consider. Check this out.

https://www.servicemaster-restoration-cleaning.com/2017/03/mold-myths/