How do I prepare an interior cement floor for painting?

Question

I have an enclosed carport that serves as 4th bedroom/den in a 30 year old house. This room has long been used as storage, etc, and is in fairly bad shape. I'm rehabbing it to serve as a home office, but so that it can still be advertised as a bedroom when I sell the house. The floor is cement (the house is on a slab foundation), and is fairly well stained with traces of adhesive where the carpet padding was glued(!) down, some stains from dogs (yeah, both), and various other irregularities. However, the slab itself is in good shape with no cracks, and a few divots from carpet tacks have been filled in and smoothed.

I live in Arizona, and painted or stained cement floors are common, and don't really have any problems regarding resale, so I would like to paint this floor rather than add flooring or carpet. Leaving the slab uninsulated helps with moderating the temperature as well. I think stain would not cover the irregularities well enough.

I've heard conflicting stories about how to prep the floor, etch or don't etch, prime or don't prime, etc. I'd like to know what the best way of A) cleaning the stains as much as possible, even if they cannot be removed completely (the paint should cover light blemishes), and B) prepping the floor for long-term adhesion of primer and/or paint to avoid peeling or cracking in the future. This is an interior room, so treatments like epoxy that are appropriate to garages and patios may not be appropriate here.

In case it matters, the room is about 12 X 15, with one expansion joint in the slab, about centered. There is little to no issue with freezing, and thermal expansion and contraction is less of an issue because of that and the fact that it is an interior room. This is also a room that sometimes in unusual circumstances gets small amounts of water seeping under the walls from outside (it's Arizona, inside vs outside is a much more fuzzy concept here), and though I am working to cut that off, it may not be entirely possible.

Answer 1

About a year ago, I prepared my concrete floors to be covered first with an overlay and then stained. My situation was a bit different, but I bet the prep needs to be about the same. You can read my blog post about the preparation we did on lauramakes.com.

For your situation, you will want to use a floor scraper to remove anything loose on the floor, like paint chips and the glue, or you won't get your paint or overlay to stick well. I found that wetting down the floor made the scraping go much faster.

You will definitely need to get the carpet glue up. I was interested in non-toxic products, so I looked into two soy based strippers. I tested Soy It, and it definitely worked for paint and is supposedly good for adhesives too.

After you get up the glue, you will need to take care of the grease stains. If you don't, it may show through your paint. The Eco Safety Products support person recommended that we use a floor buffer that you can rent to clean the floor first with orange degreaser and then water. Our grease stains didn't come completely out, but I don't think we found a heavy enough buffer to rent.

I can't really answer much about etching because I decided early on that the process was too toxic for us because we would be living in the house with our pets at the same time as we were prepping the floors.

Anyhow, I hope that helps a bit and let me know if I can elaborate on anything.