What does acid etching do to concrete?

By etching a concrete surface you are providing the adhesion needed for the paint to stick. For paint to stick the surface must be rough - it cannot be smooth or slippery in anyway or the paint will simply peel off.

Like doors and frames they need to be sanded back before they can be painted. The reasoning being that otherwise the gloss finish will not allow the paint to stick. By sanding it to create a rough finish so the paint has something to stick to.

When I say rough however - Don't misunderstand me as to think it should be really rough. The surface should still seem rather smooth however as long as its not glossy.

In the same way concrete needs to be etched or roughened so that paint can adhere to the surface. You can do this in two ways.

One is to use Hydrochloric Acid - Simply dilute it 1:10 or stronger if required and then tip it onto the concrete. Broom in and when fizzling has stopped rinse off. Make sure you rinse it off properly or it also will repel the paint.

Your other option which is becoming the more popular now (as people don't seem to follow instructions with the first and always and have problems) is to use a dinosander. Sand the surface down and rinse off - This is problem proof and easy to do. You can hire one from your local hire shop.

As I say both will work but the latter is becoming more popular as you can't go wrong.