Building a Formula for Removing Coatings from Masonry Surfaces

From D+D Online, November 2010

by Kenneth A. Trimber

Frequently during the life of a building constructed of concrete masonry units (CMU) or brick, it is no longer feasible to refurbish the exterior by pressure washing and applying more coats of paint....
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Tagged categories: Good Technical Practice; KTA-Tator; Masonry; Paint and coatings removal; Surface preparation

Comment from Jerry LeCompte, (11/2/2010, 11:24 AM)

Many wet blast applications for paint removal or cleaning can cause leaching of salts to the surface. Dry Soda Blasting, minimal low pressure rinsing, followed by a clear barrier water soluble coating (non oil base, masonary compatible) applied as soon as the surface is dry to the touch. May be topcoated or natural.

Comment from Michael EDISON, (11/2/2010, 11:30 AM)

I have heard of cases where baking soda blasting caused problems later on with efflorescence, due to the solubility of the baking soda and its migration into porous masonry surfaces. The articles does not specifically tell us why baking soda is less likely to roughen than other abrasives, but it is presumably an issue of hardness and or particle shape? There are also some proprietary systems based on abrasive cleaning using special equipment and/or media, such as sponges, and in my limited experience these can do a good job for historic work where little or no obvious degradation of original surfaces is important. I would be interested in other reader comments on this topic.

Comment from Jerry LeCompte, (11/2/2010, 12:29 PM)

Mike Edison, Please note Photo 1 above. Salts are coming from the exposed concrete block where the coating has failed. Soda Blasting does not cause efflorescence which can be observed with plain water blasting and other paint removal systems. The exact cause of efflorescence is very elusive and inconsistent. The basic reason for Soda Blasting is that it leaves minimal surface degredation with cost effective production performance.

Comment from homer hart, (11/3/2010, 9:48 AM)

Hello Ken: It is really good to know you are still here in the Coatings and related Industries. You have always been there when I and the others in the Industry need help. You have disclosed all the weapons to deal with the titled subject herein, on the Subject of removing old paint from Brick and Concrete surfaces. As,usual,you did a thorough and useful overview. Cheers & Best Wishes, Homer O. Hart, Pres. Environmental Protective Coatings, Inc.

Comment from Kenneth Trimber, (11/3/2010, 10:08 AM)

The following comments are based on the discussions initiated by Jerry LeCompte and Michael Edison. If a source of water is present, it can carry water-soluble salts from the concrete, mortar, etc. as it migrates through the substrate. The water evaporates, leaving the salts behind (efflorescence). The efflorescence will be visible as patches or streaks. Heavy efflorescence can have an appearance similar to that shown in Photo 1, although in that specific case, the white is actually traces of block filler that remained on the surface in areas of peeling (the failure was a cohesive break within the block filler). While it is possible that methods of cleaning that use water could dissolve salts and carry them to the surface as the water evaporates, the sodium bicarbonate does not play a role in this. However, the sodium bicarbonate should be flushed from the surface prior to painting. As indicated by Mr. Edison, sodium bicarbonate is less aggressive to the surface than other abrasives because of the hardness. The sponge abrasive is also less aggressive to the surface, but if the sponge alone does not adequately remove the coating, versions of the media are available that are embedded with abrasive. However, the abrasive also causes the sponge to be more aggressive to the surface. Another method mentioned in the article that is not aggressive to the surface is chemical stripping.

Comment from Tom Schwerdt, (11/5/2010, 8:24 AM)

Dry ice blasting is another option - had a test area performed on a 1937 historic bridge, and even the historic preservation folks were happy! Pelletized dry ice at 300 PSI did an excellent job at removing the (thin) old paint and all surface staining - the usual fungus/algae growth. The concrete itself was effectively untouched - even the form marks were clearly visible. From a usability perspective, best of all was the enormous reduction in debris, dust and waste - much, much better visibility than conventional or soda blasting.

Comment from Jerry LeCompte, (11/8/2010, 12:33 PM)

When cleaning concrete for painting consideration of contaminants ie. grease and oil, gum spots, will generally not lend itself to pelletized dry ice blasting while soda blasting will soponify grease and oil even when translocated by the blasting operation.

Comment from Michael EDISON, (11/10/2010, 8:42 AM)

I appreciate the responses to my questions regarding soda blasting and efflorescence. As a coatings manufacturer who has focused entirely on concrete and masonry systems for 30 years, I think I have a basic working understanding of the processes that cause efflorescence. My question in regard to sodium bicarbonate revolves around solubility and diffusion. If sodium bicarbonate has a solubility of 8.7 g/100g water at 64F, as the MSDS indicates, then wet soda blasting is potentially impregnating a surface layer of the porous masonry wall with an 8.7% solution of soluble salt. If the wall is already wet, as masonry walls often tend to be, then there is going to be diffusion of those salts deeper into the wall. At that point, no amount of surface rinsing will completely remove these salts. Then, depending on continuing moisture dynamics, drying rates, relative humidity, there is the potential for those salts to leach back out to the surface, dry out, and form efflorescence. Where is the flaw in that reasoning? It has made us very reluctant to approve of soda blasting as an acceptable means of surface preparation, when other alternatives - sponges, dry ice as mentioned by Mr. Schwerdt, do not carry the same potential risk.

Comment from Jerry LeCompte, (11/10/2010, 3:49 PM)

Mr. Edison’s point is reasonably based on educated theory. In response to his question and basis of reasoning, I must defer to histories of observations and factual reports. My experience has afforded me this opportunity. Five years with Ameron (ppg), four years Global Coatings (coatings contracting), seven years Schmidt Manufacturing (surface preparation equipment), founder of SodaBlast Systems, LLC. in 1990 (manufacture Soda Blasting and abrasive blasting equipment) retired in 2009. While SodaBlasting is recommended for numerous surface preparation applications from wood to hard metals, the only requirements that we suggest for concrete is to confirm that the coating is compatible with concrete and that the coating is not acidic since it could react with residual sodium bicarbonate and the concrete. With this said the Soda Blasting operation could be done dry then blown off with compressed air, or blasted wet using a water induction nozzle then rinsed and let dry. The conclusion to this is the fact that throughout the years we have never had any complaints of failure attributed to SodaBlasting.

Comment from Jerry LeCompte, (11/11/2010, 1:52 PM)

One question of Mr. Edison that I failed to address was “It has made us very reluctant to approve of soda blasting as an acceptable means of surface preparation, when other alternatives - sponges, dry ice as mentioned by Mr. Schwerdt, do not carry the same potential risk.” In the earlier days of discovering opportunities for SodaBlasting, I was personally involved in a building restoration project that had exposed rusting rebar, paint failing on masonry, corroding aluminum, and glass all coming together around the window areas. All areas could be cleaned and prepped in a single operation of SodaBlasting. Yes the rust, paint, aluminum corrosion was completely removed without etching the glass.

Comment from Michael EDISON, (11/12/2010, 8:38 AM)

I can appreciate Mr. LeCompte's position on the value of experience and anecdotal evidence in supporting a particular approach, though reliance on anecdotal support alone is not as strong an argument as one that is also supported by the underlying principals of physics and chemistry. My sense of the state of the masonry restoration industry is that there were sufficient negative experiences being discussed with regard to soda blasting - justified or unjustified - that many, if not most, have moved on to other alternatives. I suspect that this method has more proponents in applications involving substrates other than porous masonry and concrete.

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