You might have to save up and buy the power-washer, I agree that cleaning the windows is a bit of a hassle though... and, I'm not 100% sure about it, but I think the windows will get dirty again over time.
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The power washer is somehow worse and slower than the mop at cleaning those windows. It will only clean the exact spot where the water jet intersects the window, whereas the mop will affect a small area around the mop head. This is annoying because if you point the same power washer at an Ariral wall graffito, the whole window-sized decal will go away all at once.
One of the cheeky aliens left a “big sponge” the size of a dang watermelon outside the base where they like to scatter strange brownies and graffiti. I haven’t tried it yet because it’s a full quarter of poor Dr. Kel’s inventory – and frankly also not sure if it will explode or tempt me to eat it instead.