A coating has been specified for use over an existing concrete substrate and a field sample is required to be sure the coating not only looks the way it should, but also performs the way it should. The guidelines provided by coatings manufacturers are all pretty similar. The concrete needs to be absorbent, clean, free of chemical residues, dry, pH neutral, and without laitance. You think you have prepped properly, shot-blasting and/or grinding, and you scrubbed with an RJSC StoneClean product, but how do you really know if the concrete meets the requirements and that the coating will work. Even the best coating in the world can fail if the slab is contaminated or otherwise unsuitable.
Testing for absorbency is as simple as putting a few drops of water on the surface. If the water beads up and does not go in at all, the likelihood is that the concrete must be opened up and/or a sealant removed. If penetration is really slow, whether or not the coatings can be used will depend on the coating system specified. RJSC's TK6™ as a nano product can crawl in almost anywhere. 200-Series™ Epoxy Primer and StoneLok™ "E3" have a good chance of success even over relatively tight substrates. The only way to tell is to lay down the sample and then do the "cut and scrape" test described below.
Other than absorbency, the main issue to consider is residues because these are often invisible. Typical residues that can create bonding issues are cleaners containing deLimonene or soy which repel water-borne coatings; silicone/other impregnators (sometimes called sealers) designed to repel oil and water (which includes water-borne coatings); acid salts from an acid etch that was done before you got on the job. There's also the concern with laitance which is not always easy to recognize, especially if you have not seen it many times. (Look for a future tech note on Laitance). So what do you do?
Go ahead and install the test. It is important on any substrate to test not just the first or primer coat but the whole system. This is because sometimes the first coat will appear to bond, but when a very strong 2nd coat is applied, the bond of that coat to the primer will be stronger than the bond of the primer to the substrate, and it will delaminate. Like duct tape over scotch tape.
Wait 48 hours after the 2nd coat has been applied.
First observe the test area. If there is hazing, this could mean the temp was too low and/or the air circulation inadequate during cure. If you measured off a small volume of a 2-component system, perhaps your measurements were not exact. Or there may be a lot of moisture in the slab. First see if the coatings are hard. If not, contact the manufacturer. If hard, apply some heat to the area to see if this improves the clarity. If there is white, this may be acid salt residues so testing the bonding at this area (in addition to a clear area) is advised.
The "cut and scrape" tests for bonding are done as follows.
1) Take the dull end of a knife and push the coating. You are looking to see if the coating wrinkles up (like a heavy object being rolled over a carpet). If not, this is an indication that the bonding is secure. Now …
2) Take a knife and cut through the coating(s) into the substrate, and try to lift the coating(s). You should not get more than a 32nd of lift adjacent to the cut, and that will be because the substrate itself was broken. This result means the bonding appears to be secure. If there is a peel, even of an inch, this is a good indication of poor bonding and/or a contaminated substrate. Look at the bottom of the peel. If there are cement fines attached to the coating, it means cleaning was inadequate or there is laitance or perhaps acid salt residues. However, since it may be a spot problem, you will want to cut in 2 - 3 other locations. If all are secure, it is probable that the installation will be secure.
RJSC has some of the most forgiving coatings in the world for problematic and unusual substrate conditions, especially our newest TK6 NanoCoat. Nonetheless, on old concrete, you never know what is there, and a field test with the prep you will use for the project is absolutely recommended.