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Air bubbles and pinholes in epoxy floors: 7 tips on how to avoid them

By Akis Apostolopoulos

A close up of pinholes in an epoxy floor.

Anyone who has spent some time applying epoxies has come across this all toο familiar picture (see above). Little pinholes in the floor.

These pinholes are caused by air bubbles formed in the epoxy coating during the application process. Some people mistakenly assume that they appear later once the floor has cured. This is wrong, it's just that they may not have been that visible before.

Repairing bubbles once cured is very difficult (or nearly impossible without needing to re-coat!). Therefore the entire strategy for addressing bubbles needs to be on prevention of air bubbles rather than repairing them. Over the years I have come to identify the key factors that can lead to the formations of bubbles in epoxy floors. Some of these I learnt the hard way, meaning that I encountered a cured floor with bubbles and needed to explain to the customer. And therefore I now make sure that most possible causes of bubbles have been eliminated before applying the epoxy coating.

1. Porous slab

This is probably one of the largest causes of bubbles. No two concrete slabs are the same and when applying high build coats it is absolutely crucial that the slab has been properly sealed with primer. If you find that the slab has absorbed the primer, always go for a second coat of primer or an intermediate base-coat just to ensure that the concrete has been properly sealed off.

Porous slabs are tricky in the sense that some bubbles appear and you run the spiked roller over them to burst them. You leave the project site thinking that all air bubbles have burst, and about an hour they start to re-appear! At this stage it is impossible to go back and fix the problem since the coating is already curing. Therefore it is better to add an extra coat of primer as an insurance than rather need to re-coat high build epoxy that could run your costs up significantly.

A pen used to show the size of some bubbles in an epoxy floor.

2. Presence of humidity and moisture

Moisture can cause many types of problems on epoxies like delamination, blistering, or blushing. But moisture can also cause bubbles when the epoxy reacts with the water. Make sure that you are applying on a dry slab. If in doubt conduct moisture tests on the slab to make sure that the slab is dry. I have become a huge proponent of water based epoxy primers as they are able to capture some moisture and reduce the risk of moisture related problems (Let me emphasize some moisture, don’t assume that water based epoxies are the solution to all your moisture problems).

3. Expansion of the concrete slab

This is a tip I only learnt recently after exchanging thoughts with some of my peers on Linkedin. Concrete slabs will change temperature throughout the day. In the early morning slabs tend to be more humid and cold. Later on in the day they warm up, the slab expands and moisture is released into the atmosphere. Therefore you should avoid avoid applying epoxies early in the morning as the slab will change throughout the day and that could lead to air being released and bubble formation. Apply around 12 noon, when the slab is already warmed up.

4. Sloppy work

Sometimes bubbles are caused due to sloppy work or simply poor preparation. If you are going to apply a high build coat you must have spiked shoes and spiked rollers with you. Never leave for work without them! (Tip: always have a spare roller on hand. You never know when it can go wrong)

When applying the coat, make sure that the entire surface has been rolled over and properly inspected for emerging bubbles. Also look out for edges and corners. These areas tend to collect a lot of bubbles and they are not as visible when applying the coating. Shadows, object and reflections may obstruct your view. Good lighting is essential for avoiding bubbles.

A contractor wearing spiked shoes to roll out an epoxy floor.

5. Excessive solvent

Some contractors like to add a thinner to the epoxy to enable the product to flow better (and also to get a few extra square metres per pack!). However solvents do evaporate and they leave little pinholes in their wake. Most of these bubbles will burst on their own but there will always be a few that will not. Therefore if you need to add a thinner keep it to an absolute minimum. There is a reason why most companies sell high build epoxy coats as 100% solids.

6. Fast mixing

When mixing the two parts of an epoxy system some workers like to use a high powered mixer to get the mixing done quicker. High powered mixing though will lead to more bubbles emerging. Therefore use an adjustable speed mixer and mix at a low speed at around 300rpm. It is better to mix for two minutes at 300 rpm than 30 seconds at 1500 rpm. Tip: Let the product settle for a few minutes before pouring it on the floor.

(for more information on mixing mistakes read this article here)

Contractors mixing an epoxy floor coating with a geared mixer.

7. Poor product quality

With many low quality, low cost products flooding the market, some manufacturers are looking for creative ways to lower their costs. One way is to reduce or eliminate the additives that contribute to defoaming of the product. Another trick is to add all types of solvent in order to bring down the cost. If you are working with an unknown product conduct your tests first, as you could be in for a nasty surprise.

So now its your turn. What are your experiences with bubbles? Have you identified any causes of bubbles that I have not listed above?

Akis has been involved in the manufacturing and application of industrial floor coatings for the past ten years. He runs learncoatings which is an online resource training professionals on the use of epoxy floors. He is also General Manager of Ktisis, a manufacturer of flooring and waterproofing coatings.

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