Concrete Counter Sanding and Parging

[Dino_Pizza]

I just poured my concrete counter in the front my oven yesterday and the top came out nicely. The sides and front face require some parging near the lower portions of my 4" pour. I used an "edging/rounding" float tool along the front edge take the sharp angle away, but it was only partially successful. I'd like to round it a bit more in some places.

Will an inexpensive orbital grinder work to take some sharp corners off the counter? What grit works for this?

I'm NOT looking to turn the concrete top to a glassy finish, but can an orbital sander keep the smooth look and just make it consistent or will I have to use it on the entire counter-top surface?

If my counter front and sides need minor parging because of the wood form that was against it and a few holes, should I do it within a day or 2 while it's still quite moist?

Thanks in advance, Dino

[Tscarborough]

Re: Concrete Counter Sanding and Parging

For minor cleanup an orbital sander with 80 120 200 but be aware that you may expose aggregate by doing so. You should do any touch up ASAP after pulling the forms.

[Dino_Pizza]

Re: Concrete Counter Sanding and Parging

Thanks, I got home and parged right away. I have 90 grit sand and portland cement and added a little of some of the counter top mix I used so it would be closer in color and rubber-floated the mixture in. Seems to be an improvement. Here's the before and after pics. Small difference but still that's all it needed. I'll see after a few days of curing if the concrete colors stay close enough and if I need to do any polishing.
Cheers, dino

[kebwi]

Re: Concrete Counter Sanding and Parging

I'm so jealous of everyone else's concrete.

[Tscarborough]

Re: Concrete Counter Sanding and Parging

When you are placing the concrete, if you use the orbital sander (without sandpaper) against the form, it will eliminate those bugholes. Sealing or wetting the wood will also help. As a rule, you should remove the edge form within a couple of hours of concrete placement, as soon as the water sheen disappears from the surface. A good way to judge this is to form a test sample of your concrete while placing in (sealed/soaked) wood that is about 6x6xdepth of counter top. Remove one side at a time, and when you can pull the side off with out noticeable slumpage, it is time to wreck the edge forms on your counter top and finish the edges.

Another thing as an aside is that is is easier/better to extend the form out beyond the face, then screw the vertical form down to that surface. When you remove the vertical form, do so by sliding it along the face of the counter top.

[PlanoPhil]

Re: Concrete Counter Sanding and Parging

I recently saw an episode on DYI call House Crashers where they created a concrete table top that was smooth as glass and all they did to it was seal and polish it.
They built a mold using MDO plywood, lined it with either wax paper or plastic wrap and pored a very soupy looking cement (might of been some special formulated stuff) 1.5 inches deep. The next morning they removed the sides of the mold and picked the slab up, pealed the plastic off and voila they had a beautiful colored (forgot to say they added color to the soup) top as slick as a babies butt. Very Cool.

http://www.diynetwork.com/how-to/con...dex.html#step3

Phil

[Neil2]

Re: Concrete Counter Sanding and Parging

The right tool is an angle grinder. A 5 inch angle grinder preferably. The smaller 4 inch ones are much lighter duty tool but will work with smaller jobs.

Get a grinding stone for it. Or, for more bucks a diamond cup grinding disk.

Everyone should have an angle grinder in their toolbox. A very versatile tool.

[Jaronimo]

Re: Concrete Counter Sanding and Parging

I used a 6 inch air grinder and a hose so i could keep it wet. Felt a lot more comfortable with an air powered tool than I would have with a electric tool mixed with all that water. I was able to control the speed by turning down the air pressure a bit.

It was wet and messy, but it was better than dusty and messy. Plus the sandpaper cuts better if you use water which continuously washes away the debris.

[splatgirl]

Re: Concrete Counter Sanding and Parging

If you want to use power tools on it, you should let it cure for at least a week, possibly more depending on the conditions, or you'll just knock/tear the aggregate out of the matrix and end up with holes and deep scratches.
For changing the edge profile or overall shape of a piece, an angle grinder with a diamond cup works great. Make sure you get a cup that can be used dry unless you have a wet grinder. Depending on the age of the concrete, these can take of a ton of material riteawayquick, so be careful especially if you're using it on a fairly green slab.
If you want to polish, get a couple of diamond hand pads. They need to be used wet, and the fineness you'll need to start with depends on the level of finish you put on the slab when it was wet. A good steel troweled finish will let you start with as high as a 400 grit diamond pad. If you only floated it, you might need 120 or 200. If you want aggregate exposed, start with the grinding cup or a lower grit hand pad, like a 60 and work your way up.
I like to slurry to fill voids as soon as I can after the initial pour. With a cast in place slab, this can be almost immediately after you've stripped the form. I always leave my molded slabs in the mold to cure for a week+ and then slurry after I've set them.

[Dino_Pizza]

Re: Concrete Counter Sanding and Parging

Diamond hand pads, that sounds interesting, I'll check them out.

I filled my small holes and re-touched my "rounded" corner edges with a slurry mix of 1/3 super-fine sand (#90), portland cement and 1/3 of the quikrete counter-top mix I originally used put it thru a sieve removing 1/8" gravel out. I put it on within 2 days of the pour and now a week later, it looks fine.

I just bought a $70 Rigid HD 5" Random Obital hand sander. I bought 60 grit hook & loop (is velcro copyrighted?) and 220 grit pads. I plan to sand dry (could I add water to the counter using a "dry sander"?) and I'm NOT looking for a granite-like finish, just a smoother, shinier but natural concrete finish.

Do you think this will work? I've only got a 6' long counter just 10" deep and a 4" face. I know I'll go thru pads but I hope it'll work for this little counter. Also, I know you can get 1000 grit and up pads on-line so maybe I'll try the 220 grit (available everywhere) then see if I want a brighter polish.

thanks, Dino

[Tscarborough]

Re: Concrete Counter Sanding and Parging

You will need a couple steps between the 60 and the 220.

[splatgirl]

Re: Concrete Counter Sanding and Parging

I have always mixed my own slurry of straight cement powder and water. The sand component you added is unnecessary and I'm not sure how this will affect the finial finishing other than to say that a surface coat of anything is mostly moot if you want to polish.
A well floated and troweled slab has a surface coat of "cream" meaning a layer of 100% cement that can be polished up with very little effort and ends up being baby smooth and shiny almost all on it's own. That is the type of surface that can be worked starting with 400 or even 600 or 800 grit pads to get something like glass.
If you intend to polish your slab after the fact, you will be removing most or all of what you've just applied to the surface whether you like it or not. Since your slurry included fines (the sand), you will want to let that cure VERY well (for several days to a couple of weeks) or, as I said before, you'll end up just tearing those particles out of the cement matrix instead of polishing and smoothing off their peaks.

Right now, what you have is like unfired clay so yea, sandpaper will work on the softer elements but it's effect on the aggregate is going to be just about nothing (can you sand down a rock with sandpaper?). Personally, I would return the $60 sander and order up 3 or 4 diamond hand pads in a range of grits (check Ebay, the should be less than $10 each). Leave what you've done alone for TWO WEEKS, at which point you'll have the hand pads and can spend an hour or two with those and a bucket of water and you'll get a super smooth surface that then just needs to be sealed and it'll be shiny like glass. If you must have power tools, a $20 harbor freight angle grinder is more useful.

[Wiley]

Re: Concrete Counter Sanding and Parging

Dino, I have a 6 inch DeWalt random orbital sander and it is better at not cutting "smiles" in the surface, but will still tend to leave the surface with a gentle undulation. The pad still flexes and the softer material is cut away quicker than the harder. However, if you want a really flat surface there's a modification to the disc of a sander/polisher that allows one to sand larger surfaces remarkably flat. Although you area is small I would think it would be worth the time to build such a disc especially if you wanted a flat surface from one end to the other of your counter. And once you have one of these discs you will it quite useful for other projects. I expect these modifications will never be OSHA approved as they will leave one major bruise if you are sanding floors and manage to contact your knee (been there, done that). When one tries to tilt the sander the square corners force the sander back flat against the surface. One simply cannot cut a smile. Since the disc surface is flat and inflexible the surface is cut away uniformly, evenly no undulation. Use silicon carbide paper attached by feathering disc adhesive. Of course wear good eye protection and mask and be sure to wait at least a week for the concrete to be hard.

Hope this helps,
Wiley

[Dino_Pizza]

Re: Concrete Counter Sanding and Parging

Hey Wiley, that's a pretty cool jig on your sander. My counter is so small, I don't think I'd make one but I'm hoping my deft touch and experience in painting will somehow keep my sanding level .

Splatgirl, I used the 50/50 recommendation on this forum for parging (thats half portland/fine sand) and it's mentioned a few times here. I just mixed in the special counter mix that has plasticizers and fibers in it cuz it seemed like the right thing to do. My counter top remains untouched; just my lame attempt at a smooth finish with a new steel trowel.

I'm going to wait 2-3 weeks before I attempt to lightly polish the counter. I'm going to check out diamond buffing pads online too and I'll pour myself a square block of my concrete mix to experiment with my polishing options to see how it'll turn out.

[splatgirl]

Re: Concrete Counter Sanding and Parging

keep us posted! The fun thing about concrete is you can never completely predict what you're going to end up with. A nicely finished slab is beautiful no matter what, IMO.