There was a lot of mildew and crud because it is enclosed space that water still gets in, so this is how I cleaned it: First, sprayed some clorox down, then used a power spray nozzle to jet it right off--cleaned up nicely, but quite a bit of work. Also used a giant squeegee broom to whoosh the dirty water out. Whew am I worn out!
I have painted concrete before. See here outside the enclosed patio is a giant circular rug I painted a couple of years ago. It is constantly exposed to all the elements and has not faded or peeled whatsoever. The concrete is not smooth and I think that helped the adhesion. Also, I used a variety of my acrylic latex cans of paints and even some paint pens for finishing touches. This is a "signature" rug. See one of my other blogs FesmireArtStudio to see more about it.
I'm thinking about painting this floor with a sky---here is a sample of the colors
I will demonstrate this painting tomorrow. The weather will be nice and pleasant so I will aim to get it done. I started to go all expensive and buy concrete paint and the primer you put down, but after some research and the obvious evidence above that acrylic latex works, I will go with some paint I found in my stash---PREMIUM exterior latex enamel and another that is the same but has primer built in--this should work just fine.