A residue will usually form during the drying stage. Some acid stain colors will have a heavier residue while others will have hardly any. Regardless though, the acid stain residue must be neutralized and rinsed off with clean water. You can use mops, water hoses, wet/dry vacs, and/or extremely soft bristle brooms to neutralize the stain and remove the residue.
Neutralize with water and either baking soda, t.s.p., or ammonia and then final rinse twice with clean water only. We recommend using 8-10 ounces of ammonia per mop bucket. Or 4 ounces of ammonia to 1 gal sprayer of water.
When cleaning the stain residue, a very soft bristled push broom, similiar in softness to the brush at a car wash, can be used to lightly brush the area and help remove the residue easier.
When you rinse the residue it is important not to overscrub. The stain can be scrubbed down fairly easily at this point. Once it has been washed and allowed to dry, the stain will lock in and not be scrubbable. So if you want to scrub it down to try and lighten some areas, do it when you first wash the residue.
Actually, the removal of all the residue isn't the real objective and complete removal of the residue isn't neccessary. The reason for washing the residue is to neutralize the acid. So long as you properly neutralize and rinse the concrete well it should be fine.
Note: If you do not properly neutralize the stain then the sealer will lose it's bond, which will then have to be stripped and re-applied.
When working outside, you can use a pump up sprayer to spray the neutralizing solution onto the concrete, and then use a water hose to flood rinse the concrete.
What the concrete looks like while it is still wet from washing the residue is what it would look like if sealed with Clear Shield Solvent Base Sealer.
What the concrete looks like when it is drying down, and to the point of being damp, is what it will look like if sealed with Clear Shield Advanced Formula Water Base Sealer.
The residue will still have the capacity to stain and kill vegetation.
When mopping the residue,change the water regular.
When doing more than one coat of concrete stain, there is no need to remove the residue between coats. Just be sure the first coat is dry. Walking on wet concrete stain will leave footprints.
If you prefer it to be darker, once you have rinsed the residue, then you can do additional coats of stain.