Thicker epoxy rather than grinding concrete substrate to achieve smoothness

Question

I am covering my basement apartment's concrete floors with epoxy. The concrete is very well done (self done, been several years, no cracks, reinforced, 2-3" thick).

In order to test the epoxy, I designated my washer/dryer bay as a trial area because it is really the only part of the floor that will be covered, IOW low consequence for messing up. During the extensive conversations with a relatively helpful associate from the epoxy company, I was told that prepping/cleaning concrete is critical to the outcome. There are essentially two (as it turns out, not mutually exclusive) techniques to prepping the substrate: mechanical (grinding with a diamond blade) and chemical (muriatic acid OR a less invasive/hazardous etching solution). I opted for the concrete etch, to avoid the dust of grinding and the toxicity of acid.

So I first scrubbed the concrete really well with a rub brick and then treated it with the etch. The results were good and the removal of concrete cream (the objective of the etch) was successful. That, however, made the substrate rougher as it exposed the concrete gravel parts by removing what was in between. The epoxy company people told me that was a good thing because the epoxy needs roughness to catch on. Essentially, the substrate after the prep had the texture of laminated grains of rice.

The epoxy application, excluding the prep, consists of three layers: water based primer, solids (mid coat, the 'meat' of it all), and urethane top coat for gloss and texture. When I applied the solids in the trial area, using a 3/8" nap roller, the 'laminated grains of rice' texture remained but to a lesser degree. I understand that in a final implementation, a flooring squeegee should be used to accurately distribute a proper amount of epoxy but I didn't have a squeegee so I just used a roller. I do plan to get a squeegee for doing the final job. In order to see the difference between a thicker and thinner layer of the solids epoxy, I then applied a second coat with the roller on just one half of the trial area, so that I have something to compare it to. The results were better (less rough with the 2nd coat) but still not perfect.

When I discussed the results with the epoxy company guy, he said that I was getting such results because I didn't grind the concrete. I told him I was under the impression the whole time that etching was alternative to grinding, which I was trying to avoid by all means.

Questions:

1. Is it possible that I was not simply getting the epoxy level above the tips of the tips of gravel particles because the 3/8" roller wasn't applying enough epoxy? Would I be getting thicker coverage if I had used a squeegee?

2. The product documentation mentions a (nominal) thickeness of 3 mil. Let's say I need a little more than that to "drown" my roughness. What size squeegee notch should I use? (e.g. I think they come in 1/8 and 1/4").

3. MOST IMPORTANTLY: Is it "drowning" the roughness with more epoxy a feasible strategy to achieve smoothness, IOW a viable alternative to the hellishly messy job of grinding concrete? I don't mind buying more epoxy to avoid grinding.

I did get some answers from the company guy but I am reaching out to get some second opinions.

Answer 1

The 3 mil (0.003 inch) thickness in the product documentation is about the same as the thickness specified for an automotive epoxy primer I worked with this summer. For comparison, I sprayed that product onto steel that was prepped with either sandblasting with 80 grit media or D/A sanding with 320 grit screen. It came out fairly smooth -- certainly there's no texture from the metal showing through.

Your description of the concrete ("laminated rice") makes it sound like the chemical treatment entirely stripped the cream off the concrete and has left you with exposed fines (sand). Eyeballing a caliper here I'd guess that must be a profile of at least 20 mils. It would take a lot of coating to fill that.

The roller will lift a lot of paint back out of the depressions in the surface. A squeegee wouldn't do that, but I wonder what happens with a very thick coat. Does it cure properly? Does it bubble/blister when the solvent evaporates off the surface but more remains below? The vendor may have some guidance. You could do a test section.

It seems to me that grinding isn't inherently awful unless you do it dry. Grinding with water will make a sticky slurry or paste but zero dust. Don't leave it to dry though. Dilute the paste with just enough water to be able to push it around with a fine broom or a squeegee and pick it up with a wet/dry vac (resist the temptation to wash it down the floor drain). Apply fresh water, ie rinse and repeat, until the paste/slurry is removed. A pump-action weed sprayer or a garden hose with a misting nozzle could be a convenient way of applying the water.

Answer 2

To me it sounds like you over etched the slab. I have done many garages and basement floors and never had “rice” sized grain except where I was using really strong acid to pull out oil. Usually I use a 2 part epoxy no primer and it usually fills the voids with 1 coat, their have been a few jobs where the finish was not very good and on those I just used a second coat to fill the voids and have a super smooth floor. I am not sure what other chemical you are using to preform your etch but I would suggest making it in a weaker batch or not letting it sit as long, I have been using muriatic acid for over 40 years and never had a problem with over etching unless I planned to do it.

Answer 3

I just recently had garage and basement/mechanical concrete floors done with a polyaspartic coating. In the basement, due to the potential for moisture infiltration, the surface was sealed with epoxy before application of the polyaspartic layers. The epoxy was applied as a slurry with sand, to provide a smooth, porous surface for the polyaspartic to go on after. If you've got areas that are over-etched, the same sand-slurry approach might be a good way to fix that in your case.

I don't actually know whether my contractor ground the basement floor before applying the epoxy. The garage floor was ground using machines that nearly eliminated any mess, having vacuum hoses attached to recover almost all of the dust from the process. But from what I've read, grinding is much better than etching for prepping concrete for sealer, and I'd guess that even the basement was ground. If you can rent the right equipment, I doubt it'd be all that messy (frankly, I'd much rather deal with a dry mess than a wet mess :) ).

That said, assuming you want to avoid grinding altogether…

You should probably be using a sprayer to apply acid for etching, not just pouring the acid onto the floor. This will allow you to control the application, and make sure it's just enough to cover the surface, without over-reacting with the concrete and dissolving more than you need. In other concrete work at my house, the contractor has used a molasses-based solution for exposed aggregate, but only a dilute muriatic solution for the areas that just got a basic acid wash. The latter seems a lot more appropriate for your scenario.

Water-based epoxy (or any solvent-based for that matter) requires evaporation of the carrier for curing to complete. I would expect an extra-thick layer of epoxy to significantly slow curing, assuming it doesn't screw it up completely. I'm no expert, but I wouldn't be surprised if with a water-based epoxy, too-thick application would result in some top thickness of it curing before the carrier in the lower portion has completely evaporated, locking it in and preventing the lower layer from curing at all. It's probably a lot better to stick with the manufacturer's recommended application thickness, and apply successive layers, if you want to use the epoxy as the sole filling agent.

That said, the sanded approach the contractor used for my basement floor seems like a much better option. I.e. rather than using 100% epoxy, mix a slurry of epoxy and clean sand. The epoxy will provide structure and bonding to the etched concrete, while the sand provides the bulk and a rough surface to which another layer of plain epoxy can be applied at the manufacturer's recommended thickness (e.g. 3 mil). The porosity of the sand should allow the epoxy to cure properly, even given a somewhat thicker layer of material.

Of course, the ideal would be to not over-etch in the first place. From what I've read, that's still not as reliable as grinding the concrete, but done properly is probably fine.

Finally, consider how important it is to have a completely smooth surface at all. A little bit of irregularity could help avoid the super-slick results that a perfectly smooth epoxy floor would have when it gets wet. Without some anti-slip material on the top (e.g. more sand), it'll still be slippery when wet, but any improvement might be useful. :)