Another un-permitted, owner built, less than satisfactory condition: I had noticed the roughness of this area of the back lanai's slab, just outside the french doors that lead out to it, when we were looking before buying. I did not see it as a problem at the time. Now, after it being exposed to not being covered by a roof during my re-roof of the back lanai, it has proven to be a low area where water has pooled. Green and black algae had begun to grow and discolor the surface. I'm now considering bringing this low area up to where it is the same as the smooth areas that surround it and smoothing it out to match. It will be a fairly large thin addition to the surface of the slab. That strikes me as having many potential problems to be dealt with to obtain long term satisfactory results. The extent of my experience of this type of thing is as a kid watching documentaries that described the concrete pours done to build the Grand Coulee. If memory serves me, that entailed cleaning, etching and grinding of the surface prior to the next pour. That seems to be a bit extreme for this situation. But, the thinness of what needs to be done here does seem to complicate the job. What would need to be done to get a long term satisfactory fix for this situation?