How to Prepare A Concrete Floor For Painting

The type and degree of preparation required before concrete floor coatings depend on the type and condition of concrete, the exposure, and the coating system to be applied. The concrete should be clean, dry and free of dust, dirt, oil, surface matter and form release build-up.

Concrete floor coating can be an easy and very pleasing operation. Equally, without correct preparation, it can become a difficult and painstaking job. For maximum performance of a floor coating, the surface must be adequately prepared to effect positive and long-lasting adhesion of the coating to the substrate.

Below are the different ways that you can prepare your concrete surface.

Broom/Vacuum Cleaning

The obvious and most basic method of preparation in order to remove all surface dust, dirt, and any other loose/friable material.

Chemical Cleaning

As an alternative to mechanical methods, the preparation of concrete floors for coatings application can be accomplished by using chemical cleaning methods. Capitalized solvents or solutions as mentioned below are available.

Acid Etching

Etching is a very low-cost chemical method commonly used to treat concrete floors to remove laitance and provide a profile. The objective of acid etching is to dissolve the weak surface layer known as laitance and open the pores to allow penetration of the sealer coat.The surface is pre-dampened and the diluted acid applied uniformly over an appropriate area. The approximate spreading rate is normally around 10sq.m per litre of acid mixture. The acid should remain on the surface for two to three minutes of bubbling reaction. After application of the acid, scrub the surface with a stiff bristle pavement, scrubbing broom or rotary power scrubber will facilitate the etching process. The vigorous formation of bubbles indicates proper etching and must be obtained. The absence of bubbles or a weak reaction indicates the presence of a curing compound membrane or a surface hardener on the concrete.Once etching is completed, thorough rinsing with fresh water must be done before the surface dries – in order to avoid the formation of salts on the surface. Once formed, these salts are difficult to remove. Any residual acidity must be neutralized with an alkaline cleaner followed by thorough rinsing with fresh water.

A properly etched concrete surface will be hard but porous and will have a profile similar to that of fine sandpaper. Water will penetrate easily and will hot bead up on the surface. In many cases, more than one etching operation is required to obtain a satisfactory result. The etching solution should never be more concentrated than 1 part acid to 3 parts water. If satisfactory results are not obtained with this concentration, repeat the etching procedure. If the surface is rougher than fine sandpaper after one etching, more dilute acid solutions (dilution up to 1 part acid to 10 parts water) should be used. In addition to muriatic acid, proprietary etchants, which contain phosphoric acid and concentrated cleaners, are available and may also be used.

As an alternative to etching with hydrochloric acid, the use of phosphoric acid (which is non-volatile) should be considered in sensitive situations.

Please note that Acid Etching will not be effective over release agents, most curing membranes, or most surface hardeners. Should you wish to remove greases, oils, or other types of contaminants, these can be treated with Polycote Degrease IT prior to etching procedure.

Solvent Cleaning

Emulsifiable solvents such as Membrane Remover and the cleaner are a balance of detergents, solvents, and emulsifiers. They are soluble in water. They are powerful cleaners, which very effectively cut through and remove rubber burns, grease, oil and stubborn spoilage and hold these contaminants in suspension for easy pick up with a wet vacuum or other disposal methods. The Membrane Remover may also be used to remove many types of curing membranes. Thorough rinsing with fresh water must follow the use of these solvents. If the Membrane Remover is used, rinsing with the Cleaner must precede the final water rinse.

Degreasing

The removal of any grease, oils, and fats from floor and wall surfaces is imperative, prior to applying paint or other coatings. Polycote Degrease IT is a powerful multi-purpose cleaner and degreaser, containing phosphate, alkali builders and solvents in a highly alkaline detergent composite.Normally diluted with water at a ratio between 1:4 for floors and 1:40 for machinery, Degrease IT may also be used undiluted for removing severe grease contamination. Spray or pour and brush the chemical onto the surface to be cleaned and allow to penetrate for at least 5 minutes. The surface should then be thoroughly scrubbed to lift the grease or oil from the surface. Once done, this should then be thoroughly rinsed away. An Aquavac is an excellent way of removing the waste. Better still, is the use of a Scrubber Drier which will not only lift the contamination from the surface due to the heavy scrubbing motion but will also suck off the resultant liquid into a built-in waste tank leaving not only a well-cleaned surface but also one that is pretty much dry.As mentioned previously, it is important to realise concrete is like a sponge – it is full of air holes. Pour a bucket of water onto any unsealed concrete floor and you will watch almost the entire amount simply disappear. Being water, this will simply rise and evaporate over the following few days until the concrete reverts to a completely dry state. However, when oil or some other form of contamination has penetrated the concrete, this will never ‘evaporate’. Furthermore, it must be realised that a chemical clean will only wash the top surface. Due to the fact that the oil has penetrated into the concrete, as the surface is cleaned, the oil remaining within the slab will simply rise to the freshly cleaned surface and for the floor only to appear as dirty ‘now’ as it was immediately prior to cleaning! Should you wish to coat the floor, then this procedure should be completed as soon as possible after the completion of the cleaning. Polycote OT Primer is the best option for this as it can bind up to 15% oil into itself and its unique ability to adhere to previously contaminated floors is almost unparalleled.

Hand Tool Cleaning

Sanding and wire brushing is commonly used to remove loose matter from the surface and to a lesser extent to open voids and holes before coating. Furthermore, both power and hand wire brushing is also used to abrade the existing surface to provide a ‘key’ for any subsequent coating.

Mechanical Cleaning

The preparation of concrete using certain mechanical methods has the advantage of speed. If the more rigorous methods such as blasting or scarifying are attempted by insufficiently trained operators, however, removal of too much of the surface can easily result in exposed aggregate and an overly rough and porous surface. It must be realized that improperly performed mechanical preparation can severely damage a concrete floor.

Power grinding, sanding or scraping can be used to open holes and voids and to remove curing membranes, some hardeners, loose matter and laitance from the surface of floor slabs. These methods can be used in place of blasting but are considerably slower and less effective.

Shot-blasting

This method of industrial floor preparation is widely regarded as the most professional and most effective surface preparation method for concrete is a light blast with a Vacuum Assisted Shot blaster. The objective is the same as acid etching. The blast removes all form release agents, weak concrete, laitance, dirt, and contamination. Being connected to one of our highly powerful industrial vacuums assures that at least 95% of the dust is contained.

Scarifying/Scrabbling

When inspection and testing procedures indicate concrete floors must be chipped down to sound concrete Scarifiers and Scabblers may be used to remove the top layer of concrete to expose a fresh, clean surface. This technique is practical only for floors. Important; dust protection is required when used in the presence of machinery, food or close traffic. We have a full team of qualified operatives ready to complete this work as necessary.

Handheld/High-Speed Diamond Grinders

Far more than just a ‘sander’, our Diamond Grinders are powerful enough for almost any demanding industrial requirement. They are used for many and varied applications, from the ‘keying’ of existing surfaces, removal of existing paints and weak laitance, to the smoothing out of uneven surfaces, we have a grinder that will be ideal for your application. On the contrary, we are often asked for the complete whereby we are asked to actually roughen the surface to achieve higher levels of slip resistance.

Diamond grinding can smooth out floors to a laser level finish, removing minor pits and divots, high spots and roughness on concrete floors. Should two slabs of concrete be poorly laid to the different height, the grinder is used to smooth out the transition from one slab to the other. A huge benefit of the system is that all machines, ranging from simple hand held grinders to the biggest ride-on machines can all be connected to industrial dust recovery units. This, in turn, means that we can achieve a 95% dust free operation.

With grinders that have pressure controlled rubber mounted non-planetary grinding heads and specially designed diamonds, you are able to grind and/or polish without scouring the concrete, even on cracks and expansion joints. The specialist disc, embedded with a coarse diamond grit can easily prepare, clean and abrade existing coatings for re-coat. This process will remove thin layer coatings and weak toppings and provide a textured finish suitable for two coatings to bond together. Should an existing coating be already down that is fundamentally adhering very well to the surface but one that simply needs rejuvenating, the keying of this existing surface in order to accept a further coat can also be easily achieved.

In comparison to shot blasting, diamond grinding is relatively is slow and intensive. However, the finish left can be far smoother. In some cases, we are asked to actually polish the surface to a super smooth finish. Diamond grinders can leave swirl marks in the slab should the operative be untrained or overly aggressive.

Finally, the nature and degree of surface preparation required to achieve a sound floor substrate for application of a coating system depend upon the type and quality of the concrete, the presence of any surface treatments, the nature and length of exposure conditions, the type of protection desired, economics, and other considerations. Ultimately, the end result must be a sound surface, clean and free of contaminants and absorbent enough to ensure good penetration by the coating. No matter how good the floor coating, its effective service life will be shortened by insufficient or ineffective surface preparation.

Searching for hard-wearing flooring which combines style and substance? Ask our experts today.