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“How I Acid Stained & Sealed My Concrete” – A successful testimonial from a DIY owner-builder.

Background story

Con-Treat’s technical advisor, Andrew McLeay, was approached by Owner-Builder, Paul Fischer, who is well known for building mudbrick homes within Victoria. For his most current build, Peter wanted to achieve rustic, stained, concrete floors throughout. After consulting with Andrew, it was proposed that Peter would have great success in applying the Fortis Acid Stain product alongside Fortis 519 Polyurethane top coats. Prior to product application, the advice was also given in regards to sufficient surface preparation of the concrete floor. Below information is Peter’s detailed journal, completed with great success!

The slab

The slab is cast over plastic pods sitting on a moisture barrier, which covers 50mm of extruded polystyrene. It has these parameters:

  • The total area is 135 m2
  • Divided into four areas, including 50m2 living space, 30m2 of the kitchen, 10m2 of bathroom and 45m2 of the sleeping area.
  • When the slab was cast, the helicopter was put on it to – at least that was the expectation – float it to a very smooth finish.
  • That did not happen because they ran out of time; additionally, even some of the ‘floated’ areas turned out to be somewhat mediocre.

This is what we did

Grinding and polishing the slab.
Early quotes suggested far too high a cost to have the slab polished professionally. In the end, we did it somewhat late (too late) in the project. Nevertheless, these were the steps:

  • Hired a machine that came with three carborundum stones and ran this over the surface in three passes – the dust was substantial and the result mediocre.
  • Concluded this was not sufficient and hired what was called a Terrazzo Grinder by the hire shop, and ran this over it for another full pass.
  • The result was still very disappointing so I decided to start hand polishing; ultimately, I found that a heavy, 125mm diamond cup disk worked best and so this was run over the entire surface.
  • Next, I used 100 grit diamond polishing pads (7 inches) on a hand grinder/polisher; this gave about as good a result as I could possibly expect.

The Acid Stain being used

After a lot of research, I decided to use the Fortis Concrete Acid Stain and purchased each of the colours on offer. This is what I found:

  • Black is an inky liquid that gives about the result expected; multiple applications gave me a near black.
  • Blue, green and tan are watery liquids with the respective pigments kind of suspended in this. When the container is left for some time, the pigment will drift to the bottom.
  • Due to insufficient lime content in my concrete, these three stains we found hard to obtain deep colour, no matter how many times we applied them; additionally, the plan to have green and blue used in patches within the main colour of antique red turned out to be too hard to achieve. Therefore, I decided to only use green in a few spots to highlight a feature.
  • Antique Red: after the first application in the bathroom I decided to add about 20ml of black per liter of red, as well as a bit of the tan. And then the result was diluted by at least the same amount of water.

Applying the Antique Red stain

After the disappointing experience with the grinding and polishing, the staining job I looked forward to only wearily. This is about how it went:

  • I used a cheap weed sprayer to apply the stain in the bathroom first. Applying it was straightforward, and this produced a very wet and dark result.
  • Next day, when it was all dry, it looked very dark. As a result, I changed the way I referred to the floor that I was expecting to end up with no longer as ‘somewhat provincial’, but called it ‘Early Brothel’ because it looked so red; like what one might think ox blood to look like.
  • Then I used a sturdy microfiber pad attached to a broom to ‘wipe’ and thus rub off any further patches of pigment.
  • Thereafter it was vacuuming once more and, finally wiping it over with a large and substantial microfiber mop. This picked up a surprising amount of pigment remaining even after using a dust extractor.
  • After again revising how I thought of the outcome (no longer any early brothel), I applied the first coat of polyurethane. And this time I was pleased with the result.

Sealing the slab with Polyurethane

To seal the slab I also purchased Fortis 519 Polyurethane, aiming to apply two coats of gloss, with one final coat of satin.

  • I decided to use 360mm 10mm nap rollers to do the job, and this works really well.
  • Applying the first coat – to my great relief – brought back a very pleasing richness to the Antique Red, and added a bit of depth to a few spots of green.
  • After about six hours I decided to walk on this in my socks and apply the second coat.
  • The next day I got a new shock in that the floor looked like someone had spread sand on it while I was not watching.
  • So I decided to leave it for a day and then I did what Con-Treat recommends: sand it lightly.
  • I used a mop to which I sort of taped a full sheet of 240 grid sandpaper. And this worked well. Although I have now bought an attachment that fits onto the extensible handle that I also use for the rollers. This will take half a sheet of sandpaper.
  • Doing some modest sanding appears to be all that is needed.
  • Applying a final coat of satin Polyurethane, I then walked away and again left it for a day. And the final result is very pleasing, and probably closest to what I was hoping to achieve.

In conclusion

  • The Antique Red stain probably gives a result that is close to what is fair to expect. Although it does not look it along the way; so patience is required.
  • The nature of the concrete that you are staining appears to have a vast effect on how the stain takes – as is stressed by the manufacturer as well as the retailer of this acid stain. Reminder; the stains react to the lime content that is in the concrete, the higher the lime content, the better the stain reaction.
  • With the Antique Red, the result is highly mottled regardless of what I did.
  • With all other stains I used, the result is much more even but also much more ‘translucent’ – again, unluckily due to insufficient lime content.

“Working on this floor was quite an experience, but I am very pleased with the result.

Thank you for your assistance Andrew!”

– Peter Fischer, DIY owner-builder.

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