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

Interesting. It sounds like your efflorescence is only in discrete spots? Our concrete guy told us today that he couldn't in good conscience acid stain our floor (in a basement dig out) since the moisture level was too high and the entire floor would turn white after five or six years. So we're just going to wax and polish the gray concrete. I'm coming to terms with it.

I may gave to do that myself. We had so many issuse with our stamped concrete we are thinkiing of stripping it and staining it ourselves. As for your sealer questions: From what I have read 2 coats of good sealer is enough... My blog (though more about the problems we had with our patio) does have some useful link for maintaining and working on concrete patios. Here is a link that might be useful: ConcreteNightmare

first u have to prep the concrete, ours was new, so we could skip this step. we sprayed the concrete with the acid, we used 2 colors and sprayed the light one first, then second and brushed the acid into the concrete. let that sit for about 17 hours. then neutralized it with water and baking soda then basically scrubbed the residue off, which took about 4 hours to do. wipe concrete with white towel/pillowcase and if u get any residue on it, you have to keep wiping/scrubbing/hosing off residue. let dry completely then seal it!! all was done over a weekend: sprayed on friday nite, neutralized and residue removal saturday, sealed on sunday