The acid has to react with the minerals in the concrete, but any residue of paint or plaster will prevent or reduce penetration and reaction in that area. It will not look good. You're on the right track with the buffer and pad, but you'll need a cleaner/solvent too. When I was thinking of doing a stained concrete floor, I bought the manual and video by Gaye Goodman, which is comprehensive and excellent. She recommended a soy-based solvent called Bean-e-doo to remove all paint, glues, plasters, etc. I can't remember all the details, but she recommends at least 2 people to apply the stain. It was something like, the first person either wets an area or pours a diluted solution on an area, and the second person spreads. You want variegated, but you don't want blotchy, which can occur if you don't get a fairly even wet coat on the area quickly. She also may cover the stain with plastic bags to slow evaporation so that the reaction has a longer time to work, based on the results of her color testing. You must wear protective gear, and you MUST carefully protect walls, trim, etc. from the acid splashing. She often gets a beautiful variegated effect similar to marble, and has a technique for creating "veins" in the slab as well. I did not stain any floors because I found out our subfloor is gypcrete, not real concrete. But I can recommend her manual and video as comprehensive training from a pro who does beautiful work. She says some of the most beautiful, marble-like results are on aged slabs like yours. She explains how to seal the floor once it is dry. The most durable sealers are epoxy...but I don't know if any of these are suitable for DIY. Oil-based sealers are more durable, deepen the color, and add depth. Water-based sealers are also available. She discusses her experience with different brands and products. I have her Fundamentals manual, Artistic Acid Staining manual, and How to Stain DVD and would be willing to sell them if you are interested. I'm not here to sell anything, don't care if I do, and don't want to get banned. I'm just throwing that out there in case you're interested; I would have been back when I was wanting to stain my floors. You might also want to contact her to find out if content has been changed or updated recently. My materials are several years old. BTW, when I was researching acid staining, I scoured the web and found little info. Only 1 other book beside Gaye's materials was mentioned, and I couldn't find it. Nowadays, Melanie Royals of Modello Designs might have some information as well. She does acid-stained "carpets" with her stencils. Here is a link that might be useful: acid staining how-to

acid is best used only by pros and mainly outdoors. i have used it myself for a few projects, and it does tend to burn the skin and eyes. try to other solutions first, if they do not work then wait until the weather is nice enough to open all the doors and windows before you use acid inside. i always used a sponge to dip in the acid, then wipe the wall, soaking it. let it sit a minute or so, then scrub, then rinse. you MUST rinse it well, so be prepared for water everywhere. put down plenty of towels and change them out often.

Ask the builder what the plans are.

I'm trying to remember back when we did ours (also so cal). probably 15 years ago. we took up the old pavers, exposing dirt. then I think we poured the cement and laid this ceramic tile (could I have picked anything worse?? honestly, it was cheap ceramic tile from home depot back in 05 or so) Some of the grout has loosened in a few of the tiles, but overall, it's lasted very well all this time. I would like to completely redo this area. (i've since painted the house since this pic was taken) This shows the tile we did. There is a koi pond under the blue wall. up on the porch we did the mexican tile. (we should have chosen this for the other part, but didn't know back then) the grout could use a scrubbing. I'd suggest using a darker grout if you do tiles I water every day out here, have a lot of potted plants.