How to Prepare Concrete for Painting or Staining

Concrete need preparation before painting
Here's how to prep concrete for paint or for any type of coating. If your concrete slab has been sealed or not, it's here. We looked around the 'net and saw some "expert" DIY sites focussing on acid etching. Wow, sexy. But you need to think outside the sex.
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All painters and paint store reps agree: DIY jobs go bad mostly because of inadequate concrete prep (and moisture). The final look and durability rest on your prep, literally. How to prepare concrete for painting is very straightforward. Done right and your paint will never peel.

Concrete paint peeling problem: probably poor prep. I’m funny too. We don’t laugh at preparing concrete floor for paint.

Important points for preparing concrete floor for paint

  • Think it through: we ask important questions below.
  • Make a list of supplies you will need (including the painting phase later). Here is a starter list of tools to consider such as scrapers, rollers, brushes, etc. No tray/pan needed. More below.
  • Don’t think clean up yet: we’ll link you below to some tips. Cleaning is not a drag.
  • Start by moving out all the furniture etc.
How to Prepare Concrete for Painting: Essentials
  1. Profile? The roughness of the slab is only one key. Too smooth and nothing will stick. We explain the 2 ways to etch: etching or grinding (but you may not have to).
  2. Got moisture? Two tests you really need to do: An easy test for moisture content (all concrete is always wet somewhat), and an easier one for moisture absorption. Moisture is a real enemy.
  3. Alkalinity? Another test you might consider: new floors may be too alkaline to take any coating as is, and could require a primer. It depends on the concrete recipe used and age.
  4. Other tasks to handle (cracks? mold? need primer?)
  5. Prepainted floors need love too: Johnny get your scraper.
  6. Cleanliness prevents most failures: You have to rinse, rinse, rinse, before painting cement.
  7. Be gentle with me, it’s my first month (of curing).

Painting previously painted concrete? Jump to preparing concrete for painting if you have a painted floor.  

Just poured concrete must age a month to allow all the calcium to come to the surface. If you don’t wait, this process will PUSH your coating off the surface.

Steps to Prepare Concrete for Painting

1. Profile

Too smooth on purpose: Many builders will finish the concrete for a garage to be very smooth. This decreases the porosity of the concrete, and your coating/paint will not bond well.

If you need to acid-etch (you likely won’t), and if you don’t clean it off very well, you leave calcium dust in the concrete pores. Uber bad. This is why we rinse over and over. And over again.

At the bottom of this post is a short section on etching with the link to the full page: it’s essential knowledge but we separated it out for clarity.

If you are not sure about the roughness or profile, you need to have an expert check it. She/he will tell you that the “concrete surface profile” (CSP) needs to be between a CSP-1 and a CSP-2 level and how your slab ranks. High-build coatings may require different profiles.

Remember: acid etching will not remove oil. Cleansers/degreasers do that part of preparing concrete floor for paint.


2. Moisture

Testing is key: The water tests are easy. You need to test for penetration of water and the wetness of your old or new slab. How it’s done:

Water-drop test: A small amount of water (the size of a coin or so) should absorb within a minute into a floor that is ‘open’ to accept the epoxy coating. Non-profiled, non-sealed concrete will bead up the water somewhat before slowly sinking in and evaporating and will remain wet for some minutes.

There are other moisture tests you should know about: it’s all written in our short post on how to test concrete before painting. These simple tests are all most people do unless they suspect a specific problem.

Did you fail your moisture test?

If water won’t absorb well you have a sealant and you have to grind, not acid etch it off. Read our post on concrete etching and skip the parts about acid washing.

If you have moisture coming up, there’s a cure for that. Painting cement is fine as long as it is not too wet for painting (ask a pro to help you and/or read the post on testing concrete).

What’s the cure? Depending on the primer, it can act as a moisture barrier for floors that are built on wet soil. This can lower the humidity inside also. There’s more on primers just below.

Single component (“1K”) epoxy paints (not necessarily the 2K), are good at blocking moisture too. If you think you have a moisture problem, definitely call an experienced concrete pro. They can advise and also prepare concrete for paint.

We wrote about concrete paint here.


3. Alkalinity

Most floors, even new ones don’t have an alkalinity problem, but new concrete is questionable.

For brand new pours, never paint in the first month, and know the alkalinity of the slab: too high and you need a primer (unless your paint is formulated for that). When preparing concrete floors for painting, most people skip this aspect, which is probably fine if the slab has aged.

For this test, we send you to the concrete testing post (the same one as the moisture tests) that explains how you can do the testing yourself. Here is the concrete testing we do before painting.

An alkalinity test is shown in this video.


4. Other Considerations on How to Prepare Concrete for Painting

♦ Got cracks?

It doesn’t matter whether you treat them before or after you prime. But if your filler does require a primer… ’nuff said.

Rhino Carbon Fiber RCF-10CI Wall and Crack Injection, 10'
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  • Large gaps should not be caulked but filled with special water-tight concrete crack filler see this good filler here.
  • There is one other large crack filler we like: This product from DAP is for all intents and purposes, concrete and could be primed but does not require it.
    Dap 32611 Phenopatch Pre-Mixed Concrete Patch (Packaging May Vary) Floor leveler: not for garage
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  • For small-ish cracks, use cement caulk like this one we recommend.
  • Caulk does not need a primer. Here is our article on caulk guns.
♦ Got mold?

Mold must be killed with either bleach or some other mold killer. These days a better choice to kill mold is the EPA approved Mold Control, a safe bleach alternative that leaves a mold preventative film you can paint. No respirator needed.

A less expensive way to go (but toxic), is to wash with a weak bleach solution: bleach is a deadly poison, and protective gloves,respirator with good filters, must be worn.

We have simplified the complex world of respirator filters.

♦ Prime or not to prime?
    • INSL-X Products FLB100099-01 TUFFCRETE Floor Bonding Primer, Concrete and Masonry primer
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      Most concrete paints generally need no concrete primer.
      But new concrete may be very high in alkalinity which is unpaintable without a primer. Old slabs are different: they have had time to react with the air and the alkalinity has come down. (Always wait at least 30 days for new concrete before preparing concrete floors for painting in any case). For new concrete (less than a year old), test the alkalinity (see that section just above).
    • If you need it or if you did not test but don’t want to risk it, use a concrete primer that can handle the high alkalinity. But first, check your paint’s label: it may be formulated for high alkalinity so you won’t need to prime.
  • Our choice for a masonry primer. Our go-to concrete primer is TuffCrete from Benjamin Moore and is intended for garage concrete floors and concrete priming.

Obviously, if you prime, do the cleaning, etc., below before priming!

♦ Finally…
  • Before starting any etching or cleaning, obviously, clear stuff out if you can.
  • Mask small things with green painter's tape: it can stay on for days and not leave a glue mark (unlike the old tan stuff).
  • Mask the baseboards, walls up a foot or more (we use this 3M masker that comes with blade but some tape: nice that it hooks on your belt. Leave your masking up for the painting phase. We like the 1-inch green tape above to use on the masker.
  • We also use the masking paper with that unit, not just the plastic it comes with. One roll goes a long way.

5. Was your floor ever sealed or painted?

No? Skip down cleaning (next section).

How to paint over painted concrete floor is like painting any floor: there is no shortcut in the scraping work, sad to say. Waterproofers or sealers are the same as paint. Acid etching will not help. You will have to mechanically remove or ‘open’ the old coatings: it’s the only way, sorry.

      • Clear sealants need to come off and that means that you have to remove the top layer of the concrete: a good idea in any concrete prep. See the bottom of this post about grinding: it will link you to the full how-to post.
      • Remove loose paint with a scraper and/or a quality wire brush.
        Purdy Wire Brush, 14-inch, with Scraper Tip, 140910200
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        Don’t try to save money here because the bristles fall right out of the crappy ones.
      • Feather sand the edges of paint/bare concrete using sandpaper of assorted grits. You’ll need 80 and 100, but it’s good to have a few extra grits: assorted grits.
        3M Sandpaper, Aluminum Oxide, Coarse, 9-Inch by 11-Inch (choose assortment on product page)
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      • Get all the paint sanded: every square inch.
      • Wash the bare spots well (see cleaning below).

6. Cleaning is where many DIY’ers fail in the concrete prep

A general cleaning method would be to sweep the floor and then scrub it with the cleaners we discuss just below. We stopped using TSP because it’s too rough on the skin.

Sun Joe SPX1500 Pressure Joe 1740 PSI 1.59 GPM 12 Amp Electric Pressure Washer
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Pressure washing is fine if you did not just acid-etch or grind. Before doing any pressure washing, you do not want tiny particles on the floor. The pressure can drive them into your floor only to work loose later. If you want the best value in pressure washers, this Sun Joe will never die. Ever.

Key point before painting/coating: Let it dry totally. A fan and space heater is very helpful here, especially if you want to do the work then next day. Again, getting it as dry as possible is key in preparing concrete floors for painting.

Particles of concrete will not be all washed away so use a wet/dry vac and soak up water. You can hose more water as you vacuum to help you get it all the stubborn bits.

Concrete cleaning preparation for paintingWhile you are at it, clean the first few inches of your driveway because the dirt on that will keep your masking tape from sticking later on.

Do your cleaning in this order:

  • Get a large part of impurities with just water: Hose or use a bucket, then vacuum, then mop, or wet-vac again. We do this twice before applying the cleansers below.
  • We always prefer to flood the floor with a hose and squeegee the water down a drain if there is one. Be sure the drain isn’t clogged first!
    Oil Eater  Original 1 Gallon Cleaner/Degreaser
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  • We suck up water with wet/dry vac. We like this wet/dry vac for power and portability. Cleans carpets too.
  • A must: clean the concrete with a degreasing solution. This removes all the oil and grease etc that will prevent the bonding of your paint/coating on the concrete. We really like the non-toxic Oil Eater degreaser because we can use it in different strengths all over the house. Use full strength to prepare concrete for painting or staining, but dilute for general cleaning.
  • We also really like GP66 Green Miracle Cleaner: a real American success story, “Since 1966”.
  • How to apply the cleansers is in a post on our 3 top concrete degreasers/cleaners. Each is about the same, but it’s a quick read.
  •  We use a stiff brush to push around the cleanser: the brush is linked just below.
  • The traditional concrete cleaner is trisodium phosphate (TSP) which is too toxic for us, but brings all the grease, etc., right out of the slab. Our old TSP recipe:
    Savogran Trisodium Phosphate (TSP) 4.5lbs, 10622
    ×
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    1. Mix 1/4 cup TSP with 1 gallon of warm water in a bucket. Stir well until the TSP dissolves. Double the TSP if it’s extremely greasy/oily.
    2. Pour the solution onto the concrete in sections and scrub it with a stiff brush or broom. Our brush can be used with a pole, and wash the car tires later. A painters pole works with paint rollers that has both the threads or the snap-lock style attachments.
      Weiler 44026 Palmyra Fill Deck Scrub Brush with Wood Block, 10" Overall Length
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    3. Rinse the concrete: a hose is fastest. A mop and bucket work just fine too. The wet-dry shop vac linked below is best because it brings up dirt that a mop cannot.
    4. Tip: suck up water with wet/dry vac. We like the wet/dry shop vac shown here for power and portability. Cleans carpets too. A slightly less expensive one is linked in the next section.
  • Try to get 99.99% of the rinse water each time you rinse because the rinse water contains loose concrete particles. If you leave on them on the floor, they will get into the paint. This makes that spot easier to chip, especially in the traffic lanes.
    Shop-Vac 16 gallon 6.5 HP Wet Dry Vacuum Long Life Motor, Uses Type U Cartridge Filter & Type R Foam Sleeve & Type G Filter Bag 5987400
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  • Rinse several times until you are sure it cannot be cleaner. Not doing this can make the paint bubble or delaminate, and become a monster that keeps you up at night.

7. It’s drying time again, you’re gonna cure me
Stanley Wet/Dry Vacuum, 8 Gallon, 4.5 Horsepower, Stainless Steel Tank
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Let the slab dry totally: this is trickier than it sounds. Moisture is the most common cause of coating failure. If it is not totally dry, you very well may have bubbles or delamination later.

A wet/dry shop vac is very useful when rinsing your floor, not to mention the many other uses you will find for it. Mopping cannot get all the little bits.

    • A wet/dry shop vac really helps to speed the drying.
      STANLEY 655704 High Velocity Blower Fan - Features Pivoting Blower and Built-in Outlets Floor Dryer
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    • Be patient and let the concrete dry completely. Fans will help.
    • The floor blower you see here has almost perfect ratings. Electrical outlets on the side: good idea, Stanley.
    • Finally, if you have one, a dehumidifier is the best way to dry the slab quickly and to the fullest extent.

 


 

Concrete preparation- grinding before paintingConcrete Prep: Special Section on Etching

To etch or not to etch, that is the question… Whether ’tis nobler to GRIND THAT BABY

Despite what other websites say, most concrete floors do not need etching. If the concrete is very smooth (some builders trowel it smooth) then yes, you need to etch either with acid or with a floor grinder. Yow-sa.

Should you grind? It is often recommended as many concrete slabs, especially when trowled smooth have a layer of ‘laitance’. Trowling brings moisture up to the surface. This creates a weaker top layer that should come off. Grinding is the only way.

Test: Use a screwdriver type thingy and try to try to make a divot. If it’s not easy, you don’t need to grind. If a layer/chunk jumps off, you really should. After that, clean (more below) and paint.

Bosch Angle 5" Grinder Variable Speed with Paddle Switch, GWS13-50VSP
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To find out if you are one of the lucky ones that need no etching, switch over to the quick read on etching (linked just below). It explains what acid will not do for you as well.

We separated the etching post out for simplicity. Start with the self-test for your concrete. If you need to etch you’ll find the full “how-to” acid wash or grind instructions right there.

Concrete Etching: Grinding or Acid Wash. It’s a 10-minute read.

Just a few key takeaways from that page:

  • etching will not remove the oils
  • wear a respirator! and other body protection including goggles
  • rinse extremely well
  • the end result should be a floor with a feel of 100- to 80-grit sandpaper

DEWALT DW4774 4-1/2-Inch Double-Row Diamond-Cup Grinding-Wheel grinder
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We don’t rent floor grinders. We prefer an angle grinder used with a diamond edge grinding wheel shown. For nearly the cost of a rental, you get to keep the very useful tool…for when you paint your house: ouch!

Summary

Prepare the concrete very well and you should have no problem. The homeowner who does this alone makes one of two typical mistakes. The slab is left wet or is getting wet in some way, or the slab is dirty/oily. To prepare concrete for painting means it becomes clean and dry. The list at the top of this post are the key points to adequate concrete preparation for painting or any coating, so please remember them! If we can advise you in your preparations let us know below.

Was this helpful? Any comments? Please feel free. Have you started preparing concrete floors for painting? Any questions?

 

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