Last updated on
When pouring concrete walls and beams, that will be exposed for architectural purposes it is rare for all concrete to be 100% perfectly smooth. There are always ways to improve the finish though by using different building methods that can help reduce concrete imperfections.
After the concrete has set, you might need to sand concrete by hand to tidy some areas up. And also use a cold chisel for cleaning any messy corners.
The best way to fix concrete and sand a smoother finish is to use an orbital palm sander with fine-grit sandpaper, or wet and dry sandpaper for small specific areas. After sanding a few coats of acid wash will help remove sandpaper scratches.
The best way to remove unseen bumps on a concrete surface is to use an angle grinder equipped with a masonry grinding stone.
Before sanding concrete
- Is the concrete going to be exposed and seen?
- Is an entire concrete surface going to get grind ‘ed down like a concrete floor for example, or do only small and specific areas need to be sanded like on exposed beams or walls?
- Is the concrete going to get a polyurethane finish, a natural finish, or acid wash finish?
- Has there been any coloring added to give a different shade or tint for architectural purposes
- Are there holes that need to be filled
How to sand concrete [Exposed]
Tidy corners and large humps
How to get a perfect finish on concrete? when you have a concrete architectural finish it is important to get the concrete surface as close to perfect as you can. At the same time, you don’t want to leave scratch marks behind from sanding. Following the steps outlined here, you can fix concrete irregularities.
You want to clean the overall concrete surface from concrete bumps from concrete that has made its way through form-work joins, from pressure and vibration.
First, get a cold chisel, and clean the corners, these bits of concrete usually break off fairly easily with just a hammer and cold chisel. It is best to do this soon after a concrete pour, as the longer concrete is left to cure it will become harder for these bits to break off.
Sanding concrete using an orbital sander
If you have concrete that is going to be exposed architecturally, you can sand it by hand using wet and dry sandpaper or an orbital sander. After sanding you should apply 2-3 coats of “acid wash” on the entire surface to hide sandpaper scratch marks from the concrete.
What type of sandpaper should be used on concrete? It’s important to choose fine-grit sandpaper, that leaves small marks, to speed things up you want to use an orbital sander.
For small areas, you want to use a fine grit wet and dry sandpaper. You might then not have to use an acid wash on an entire wall.
After sanding, concrete scratch marks will be left behind with a different look to the original concrete finish. For this reason, if concrete work requires sanding, the whole concrete surface will need 2-3 coats of acid wash to clean the surface by helping remove the scratch marks.
Sometimes concrete has a colored tint, sanding marks can be more noticeable even with fine-grit sandpaper as the color will fade on areas getting sanded. This diminishing of color can be fixed by acid washing the entire surface.
How does acid wash work? An acid wash will eat of a thin layer of concrete, this will give a new and clean concrete surface with a slightly darker shade.
When applying an acid wash, take care by wearing safety glasses, and a bodysuit. Because the acid leaves a high amount of fumes you want to make sure you have good ventilation, use a fan if indoors and open all the doors. Take breaks, and use a mask if necessary.
How much acid wash needs to be applied? There is no min or max, just do it until you are happy with the concrete finish. You should apply 2-3 coats of acid wash using a paint roller. Wait the designated time and then shortly wash it off using a water blaster.
Can I use an acid wash on masonry blocks? Yes, acid wash actually has a huge effect on how they will look afterward compared to just concrete. Acid wash really eats in they will look almost like they have been grind’ ed and had a coat of polyurethane on. Therefore acid wash is a much cheaper option for grinding masonry blocks for garages or even living areas. It could arguably even be a better look, depending on your preference.
How to give concrete a clean wet look? Applying a few coats of a polyurethane epoxy finish will give concrete a wet look. Polyurethane is mostly used on polished concrete floors, driveways, masonry blocks, and concrete tilt slabs. I like the Polyurethane look on concrete as it looks cleaner.
Does concrete with polyurethane get watermarks? It shouldn’t, polyurethane finish on concrete will stop watermarks that unevenly occur with dew or rain.
Polyurethane can also make it easier to clean off graffiti. Concrete and masonry blocks are very absorbent and can absorb paint from taggers, specific anti-graffiti polyurethane is recommended in areas that are possible to get graffiti.
Form-work tips for best concrete finish
The main cause of a bad concrete finish is not correctly vibrating it to remove air bubbles when the concrete was being poured. Either that or unevenly built form-work.
A cold joint is a join between different concrete pours for example, the join between a foundation and a column or beam, this is a weak point in concrete joins, and construction.
The most prone spot for badly finished concrete is the bottom join of a column, you really don’t want any air bubbles stuck at the bottom between concrete joins.
Bogging form-work joins
Builders generally use pink builders bog on form-work joins in an attempt to hide these joins on the finished concrete surface. This is generally overdone as bog ends up sticking and staining the concrete in the end. This is more of a pain to remove with sanding rather than just sanding the finished concrete join without the bog there.
What is the best way to Join form-work? The best way to finish exposed concrete is to make a feature out of joins using angle fillets. Or using a dedicated clear form-work silicone this is easier to remove than builders bog and does not leave a pink stain.
Although you still need to use bog on uneven surfaces as silicone cannot be sanded flat like bog can.
Pre-pour cement motor mix
As discussed above the most common reason for defects to occur in concrete is air bubbles that get trapped at the very bottom of a wall or column. An easy trick to help this from occurring is to mix up a runny high “MPA” cement-based motor mix, and adding one bucket of this at the bottom of each column or wall, minutes before a concrete pour.
Having a bit more liquid at the bottom will help air bubbles escape when vibrating, and the extra cement will compensate for the extra water. It is also important to vibrate concrete with the correct technique for structural and visual reasons.
Vibrate concrete well
When vibrating concrete trapped air bubbles are drawn towards the vibration. Some builders think a concrete vibrator just moves the mix around but most importantly remember that you need to remove the air bubbles that are trapped. Therefore you need to use the concrete vibrator to guide the air up to the surface so the air can escape.
- Hold the vibrator at the very bottom of the formwork for a good 3-5 seconds. Let the air struck at the bottom, be drawn away from the edges and towards the middle.
- Slowly draw the concrete vibrator up towards the top, you will see all the air bubbles surface when it nears the top
- Repeat this a second or third time, if reasonably sized bubbles are still popping on the top surface.
- Don’t overdo this as vibrating also brings more water and cement to the top at the same time.
- Do this in sequences of 1.5 meters at a time, so you are not doing too much at a time.
- You can also use a hammer drill with a wide chisel bit. To do this use the hammer drill directly on the formwork from the outside and vibrate any areas that are likely to have bubbles stuck. This is especially useful for any form-work that has design patterns, as air bubbles can get trapped in these areas easily.
How to grind and sand concrete quickly
How to quickly sand concrete that is not visually seen? If a concrete surface is hidden and not exposed it would be silly to use an orbital sander as you would for exposed concrete as this is very slow. The quickest way to sand concrete is to grind it with either a 4.5 inch or a 9-inch angle grinder equipped with a masonry grinding stone.
If you need to do more than a small room hiring a commercial floor grinder will save your back and time. A bosch angle grinder can easily and quickly remove high spots and humps on concrete surfaces and concrete floors, but to flatten of large areas for polishing you should use a commercial-sized floor grinder.
How to use an angle grinder to grind concrete
First, wear a mask as concrete dust is not good for your lungs. Cover household things up with drop cloths as dust will go everywhere.
Have a second person holding a vacuum if possible while one person is grinding to reduce the mess. Or better you can get a dust control attachment for your grinder.
Position yourself so that your legs are reasonably far apart as to keep your back straight when you bend over. And make sure you use an RCD at the power source.
I generally recommend using a small angle grinder in place of a 9-inch grinder as a big angle grinder is more dangerous, however, using a 9-inch angle grinder with a grinding stone is relatively safe compared to cutting steel as there isn’t a blade that can jamb and kick-back.
This makes any size angle grinder, even a 9-Inch grinder pretty safe for sanding down concrete floors. For this job the extra power of a 9 or 7-Inch tool can make the job quicker and easier, however, you can still use a smaller 4.5-Inch grinder if you already have one it will just be slow.
Bosch makes the best angle grinder for grinding down and cutting concrete, have a look at my angle grinder review.
If you are grinding more than just a small area like an entire living room, for an exposed-concrete-floor for example. You are much better of hiring a commercial unit this will save your back and be much quicker, most importantly you will get a flatter surface more easily. An angle grinder is only good for removing small obvious humps in concrete.
And finally, for working above your head it is easier to use a Dewalt cordless grinder instead of the corded Bosch that will be heavy with a lead in your way for over-head work.