Before Painting: Concrete Floor Prep

Part Three About Painting Concrete Floors

Before you can paint, you need to prepare the floor properly so the paint will adhere to the floor. These instructions are for indoor living spaces, they are not adequate for a garage or workshop.

First move the furniture out of the room so you will have ample space to work on the floor.

A) Pull up carpet and padding. While pulling it up, cut the carpet into strips of a size you can handle. Roll up the strips of carpet and duct tape the roll to make it easier to haul to the dump.

Pry up the carpet tack strips with a small nail puller or a hammer claw.  Pull out carpet nails with the same tools.

Sweep and mop often between each of these steps.

Carpet adhesive and plaster - filled nail holes.

B) Clean up and then patch nail holes and cracks with pre-mixed concrete patch. Some mixes work better for holes, some are better for tiny cracks. Use epoxy crack filler for larger cracks. The epoxy will stretch as the crack widens, so your paint will last longer before a crack shows through. If I had it to do again, I would finish the concrete patches over large nail holes with a topping of epoxy for a smoother finish.

Some older nail holes from a previous carpet installation had been filled with white plaster.  I chiseled and wire brushed the plaster out of the holes.  Then I filled the holes with concrete patch.

Just mopped.  Cracks have been patched.

 While the patches are drying:

C) Remove the carpet adhesive. Apparently, different solvents and methods work for fairly new, still sticky carpet glue than works for old, dry, hard carpet glue.  For new, sticky carpet glue visit http://www.vintagerevivals.com/2013/06/how-to-paint-concrete-plus-my-secret.html 

Removing old carpet glue: I tried all kinds of things.  A number of solvents are supposed to remove all the glue, but none of those I tried were effective, and the odors aren't so great inside the house, even with plenty of ventilation.  Of these, acrylic paint thinner, which contained xylol, worked fairly well.

Better than the rest, with less odor, and no skin irritation, was the regular Krud Kutter for cleaning and degreasing.  Don't use the Krud Kutter glue remover if you want to put a transparent finish on your concrete. That stuff stains the concrete and does not do any better job.  Also the greasy stain might cause a problem with adhesion of the paint.

Once the Krud Kutter has worked for a minute or so, you still have to scrape the glue off the floor. Use a floor scraping tool. I was loaned a tool that looks like an over size wood chisel, with a 2 inch wide blade and about 1 1/5 ft. long. It stayed sharp and had the right length to get good leverage for scraping off glue and paint.  (Paint came off like a charm after the Krud Kutter had worked on it.)

It is important to have a real floor scraper, because it will hold it's sharpness better than a hoe or a wood chisel.

If linoleum adhesive is all over the floor, use a long handled floor scraper, which looks like a straight hoe, to scrape adhesive off the floor.

The carpet glue has seeped into the pores of the concrete, so you will not get it all off. You will have to scrape and then sand it down as far as possible.  The scuffing of the sandpaper will help the new paint to adhere to the remaining glue.

In searching for something to get the epoxy out of the pores I learned that one can burn off epoxy, which burns at a far lower temperature than provided by a small propane torch, but I found it extremely time consuming to burn just a small amount.  ( Note: I read that burning super glue releases cyanide gas, not good!)

D) Sanding the floor: You can use a pole sander with 80 grit paper. It saves the back.  You also want to sand the whole floor to open up the pores and avoid acid etching.  That is, unless you enjoy the hazards and fumes of acid and are applying an acid etch stain.  In my opinion, those stains are best applied on a garage floor or outdoors.

For the living room, I used a belt sander with a light touch. 80 grit sandpaper will sand those patches smooth and sand the strips of glue, as well as scuff up the whole floor. If you have the physical strength and the money to throw around, you can rent a floor sander. But I like the quick results I get with a belt sander. I bought a cheap sander, under $60. I figured if it lasted long enough to sand the floor, it was good.  The draw back is a back ache while using the belt sander.  One also has to be careful not to over-sand with a belt sander.

Use a block sander, pole sander, or orbital sander in the corners where the belt sander can't reach. Use ear protection, safety glasses, and a good dust mask or respirator. Where possible, stuff rags around doors to keep dust out of other rooms.  Keep your sensitive electronics away from the dust.

E) Sweep and mop yet again. Now it is time to use TSP.  Use the small amount recommended on the package, mixed with water.  Use an “acid brush” on a long handle to scrub the soap or TSP into the floor. Then mop it up. You will have to mop and mop and mop. A Depression Era rag mop is really useful.

Some people say dish soap does the job on a living room floor, since there are no automotive greases in the concrete.  I tried dish soap on a bedroom floor.  I think I prefer TSP.  It seems to do a better job of cleaning, which is important if the paint is going to adhere.

I mopped for an hour or two each evening for a week until the water was as clear as it could get.  A year later, the paint is holding up well.  However, I do not have small children running through the house.

Next post:  Painting the Floor