FenTiger

We have two concrete walls, about 0.5m high at front of property. Both stained from grime, whatnot, etc. Tried to hand scrub but not effective so got out my karcher jet wash. Had to use one attachment which focuses water pressure on a very small area. So far one side of a wall is looking alot better. But once both walls jet washed should I apply a protective coating?
Tried to upload photos showing how bad it looks and the jet washed wall but can't upload.

Pulaski

IMO any coating that traps moisture in the wall is just going to invite frost damage to the surface.

If the grime you're scrubbing off is mold and algae you could spray it with dilute bleach to discourage regrowth, but you'd probably need to reapply it a couple of times each year,

FenTiger

Ah, never thought to use diluted bleach. Will try tomorrow. I bought Karcher patio cleaner liquid but not effective. I actually tried hot water, washing up liquid and a scrubbing brush but it didn't budge!
Last time I scrubbed "painted" walls was in UK. It didn't end well. After a hard day of scrubbing "painted " walls I had one beer too many and left my dog outside. Fell asleep! 2am a police officer was standing over me. I thought WTF .... neighbours complained my dog had been barking from midnight to 2am! Neighbours thought I was dead! My wife was upstairs with our other senior dog! Police smashed our conservatory door .... cost £2,000 to replace because it was Easter weekend. Luckily claimed from insurance.

Jack_Russells4ever

+1 on the diluted bleach if it is algae, It does need to be done a couple times a year. If it is a paintable surface you may be able to find an algae resistant paint but if it does not allow the wall to breath properly I wouldn't use it.

FenTiger

I learnt a harsh lesson in UK using paint which doesn't allow the wall to breathe.