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Surface Preparation - Can I Sand Concrete?

By Jack Josephsen

Is sanding an option for preparing new concrete slabs? I guess so, but not if you want the coating to stick!

This is very much “Floor Preparation 101” and I don’t want to spend too much time on it, but I still hear of contractors using coarse sandpaper on a Polivac machine to prepare new slabs and thought I’d explain why it’s not a great idea once and for all.

(Note: For those unfamiliar with the term, Polivac is a particular brand of machines used for buffing and sanding.)

Sanding concrete doesn’t remove laitance

I think this bad habit can creep in when contractors come across a fresh, clean slab and underestimate the work that still needs to be done to make a coating stick. The slab might be brand new and a quick sand may seem good enough, but the problem with doing that is it fails to remove all the laitance and that means you can be headed for big trouble. The fact is sanding in this manner might only remove around 5-10% of the stuff sitting on top, which means the coating will be floating on a bed of weak powder rather than anchored to a solid surface. It won’t be bonded well and will probably peel off as soon as it’s put under any stress, such as a car parked on top of it.

While preparing a new slab is generally a much easier task than a damaged, contaminated slab, you still need to do it properly. In most cases, this all boils down to laitance and getting the job done there. If you don’t, expect a call to come back and fix it! 

A Polyvac machine that looks similar to a grinder, but is commonly used by installers to sand rather than grind.

When can I use a Polivac?

While using sandpaper on a Polivac doesn’t adequately remove laitance, you can get attachments that do the job well. As I mentioned in a previous post on how to do the polished concrete look with a clear epoxy resin, an example of this type of thing is something called the Diamabrush Concrete Prep Tool, which uses diamonds on flexible brushes to prepare the concrete. As discussed in that post, it’s actually a good option for decorative epoxy flooring because it follows the contours of the slab rather than flatten it completely and that can have a number of advantages.

Like I said, this topic is well documented, and I didn’t want to go too far in this particular post. I simply wanted to highlight this common problem once again and ensure that you don’t feel tempted to sand the floor. There are many ways to remove laitance, however sanding is NOT one of them.

You can see more about the different types of surface preparation for epoxy flooring in a separate know how article.

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