Why did my concrete paint cover less area than expected?

Question

I'm painting my patio/back porch and I ran out of paint. It's 28 sq m of 10 year old concrete. Prep included pressure wash, mold scrub, rinse, dry time, sweep and vacuum. Etching not required as it's rough like pavement.

I bought 4L of White Knight Ultra pave which should have covered 40 sq m. I thinned it with 20-25% water. But I ran out of paint with a good few square metres unpainted and in some sections the paint is quite thin

I'm quite shocked that the coverage is about half what it claims.

Do I now buy 10L? Perhaps the second coat will go further as less will be absorbed by bare concrete? But it won't be thinned with water so maybe it won't go as far??

After I finish this first coat, I feel I'll need to do the second coat with a brush to get it in all the little crevices and tiny cavities that the roller can't get into. Then I guess it will need a third coat with a roller to cover the brushstrokes.

So it seems like another 4L won't be enough so I should get 10L so in total I'll be spending nearly $400 on paint which seems ridiculous for just the patio so I'm wondering if I have this all wrong or what I did wrong

JacobZ · Answer

Hi @KyAnne,

Welcome to the Bunnings Workshop community, it is wonderful to have you with us.

Did you have any photos of the concrete that you could share with us? Seeing what you are working with could certainly help us to understand the scenario a bit better. Let me know if you need a hand uploading images, I am happy to assist if required.

Keep in mind that coverage rates are estimates only and these estimates are for a single coat.

White Knight state that the coverage rate for their Ultrapave paint is "Approximately 10m² per litre, depending on the porosity of the surface." and based on your description, with the concrete being unsealed and 10 years old, I imagine it is going to be quite porous. The other factor that adds to this is roughness, as rough or heavily textured surfaces will typically require significantly more paint than a smooth surface because there is a much greater surface area to coat due to its peaks, pits and texture.

I don't think you have done anything wrong and everything you have said tells me you have read the instructions and are doing the right things; I just think the concrete is likely not ideal for minimising the amount of product required.

The first watered down coat is essentially acting as your primer in the absence of one, as thinning it out will help the paint to penetrate into the pores of the concrete and create a seal for which subsequent coats will stick to. Subsequent coats will not penetrate into the concrete as much as there is already a seal from the first coat, so you can expect better coverage on that second coat.

From here, I would as you have said, use a brush to apply a watered-down mix to any of the cavities that the first coat missed, but not to the whole surface area. I wouldn't really call this a second coat as it is more of a touch up than a full coat. Once dry, apply a second coat, that is not watered down, to the entire area, using a brush to apply it to any of the cavities that your roller might miss.

Considering the first coat will seal the surface and remove the porosity that is likely a major reason for the reduced coverage rate, another 4L could be enough, but again, it is tough to say with variable conditions and erring on the side of caution, I'd say you'll need a bit more.

It might be wiser to grab a 10L bucket of paint, so you have some spare. It's unlikely you would use all of it, but you can always seal it up tight and store it in a cool dry place to apply a subsequent coat in future to brighten things up a bit.

Let me know what you think.

Jacob

twocutekelpies · Answer

Hi @KyAnne

I was talking to a Bondcrete rep the other day and she said that this is ideal for use as a primer but only under water based paint which is what you are using, perhaps try this on the area you have left to go before you put on any more paint.

From their website -

Concrete Primer/Sealer

One sealing coat of diluted Bondall BondCrete (BondCrete 1:Water 4) will provide an excellent sealer or key when applied to new or old cement rendered walls, plaster, etc. before using water-based decorative paints. Allow 24 hours to dry prior to painting with a Waterbased or Acrylic paint.