Anyone Ground Concrete before?

I'm gonna have to do an 80-grit grind on my garage floor soon to put down coatings- the floor is 21 by 23 feet. What should I look for in renting a grinder? What size? How-tos? ect.

I did it when I moved in. Rented a machine with a diamond grinder from Home Depot. The amount of dust is unbelievable. Also NEVER stop, always keep moving. For the corners you need a 4" grinder with a diamond cutter on it. 

Thinking back I would have done it all with a 4" grinder. I have a 660 sq ft garage. The HD was a pain in the ass to transport as it was huge. 

Also if you are doing epoxy, buy the spiked shoes. I wish I did as I could have touched up a couple of spots. 
Also epoxy, buy 2x what they recommend. 

I rented a diamond grinder too. The dust is crazy and gets absolutely everywhere. If there's an option for dust collection from the rental place, it's worth it. Protect your eyes, ears and lungs.

If you've ever used an orbital floor polisher, it's a similar technique. Keep it in front of you, and moving side to side in an arc pattern.

Also agree that spike shoes are worth it. They're cheap on amazon, or old golf spikes from the local thrift store might work too if you get lucky and they have your size.

If it's a two part epoxy, plan your attack and work efficiently. An 18" roller really helps to work quickly.

The best machines are built by EDCO. They come in single a dual head styles and yes, they are heavy.  You can spray a bit of water to keep the dust down.  EDCO has some good videos on their website.

Knock down any big bumps with a hand grinder if the machine you get uses the grinding blocks that are held in with wood wedges. These machines were first used with actual stones for grinding and they shifted to diamond inserts later.  You hit a big chunk sticking up and a grinding block will get knocked out.

So far it seems like this is the best option in terms of sheer ooptions, but I am not sure if I just attach my shop vac to it and if I get multipule grinding wheels.

https://www.honeymanrentall.com/equipment.asp?action=category&category=18&key=035%2D0045

I've done lots of patching so there's lot of raised edges; should I just use something like a hand-grinder to knock them down before using the floor grinder?

Slippery (Forum Supporter) said:

I did it when I moved in. Rented a machine with a diamond grinder from Home Depot. The amount of dust is unbelievable. Also NEVER stop, always keep moving. For the corners you need a 4" grinder with a diamond cutter on it. 

Thinking back I would have done it all with a 4" grinder. I have a 660 sq ft garage. The HD was a pain in the ass to transport as it was huge. 

Also if you are doing epoxy, buy the spiked shoes. I wish I did as I could have touched up a couple of spots. 
Also epoxy, buy 2x what they recommend. 

Agree on all of this. At least 30% extra.  We ran short on flakes for the last 30sf or so. 
 

What you need depends on what is there now. I have recently done both the garage and back porch of my new to me house. Garage had one coat of crap concrete paint on it, came up fairly easy with the edco from HD. Porch had many layers of paints and stains, did that with a 7" grinder and diamond turbo cup wheel from HF. The turbo cup generates a metric E36 M3-ton of dust.  It took me many hours over a few days bent over in PPE to do the porch, and the smaller grinders are hard to get a flat surface with. 
 

used Supercoat Decopoxy from Amazon on garage floor, pretty happy with it so far. 

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MattGent said:

What you need depends on what is there now. 

There's nothing; just stains and now, patches that have fully cured.

Then I'd rent the Edco from HD. And acid etch. 

MattGent said:

Then I'd rent the Edco from HD. And acid etch. 

The HD rental is 10" but nearly $150, for only 4 hours. That's not even getting into what grind it'll do, or if I'll need multiple stones I'm afraid.

I did the garage shown in 4 hrs. I forget which pad but they will recommend one. Like this https://www.homedepot.com/p/Diamabrush-16-in-Coating-Removal-25-Grit-CCW-with-NP9200-and-Riser-92160120210/206926861?source=shoppingads&locale=en-US&mtc=Shopping-B-F_D25T-G-D25T-25_7_POWER_TOOL_ACCESSORIES-Multi-NA-Feed-PLA-NA-NA-PowerToolAccessories_PLA&cm_mmc=Shopping-B-F_D25T-G-D25T-25_7_POWER_TOOL_ACCESSORIES-Multi-NA-Feed-PLA-NA-NA-PowerToolAccessories_PLA-71700000034127227-58700003933021549-92700031755124916&gclid=Cj0KCQjw-uH6BRDQARIsAI3I-Uc_9X2ETXyq9GNWbH-yHoNCIV6ipnEm3zmBuh_JRfw2JTK8gI_o6b4aAsSPEALw_wcB&gclsrc=aw.ds

I had over 16hrs, maybe 20, in grinding my 12x24 porch with all the layers.  Plus the cost of the grinder and wheel. And it's lumpy.  I won't do that job that way again. 
 

In your case I'd worry more about the stains, if they are oil, than the finish. 

That wheel above is for removing paint and various coatings.  The grinding blocks have segments of diamonds like ladder rungs.  If you are trying to smooth out concrete you need the blocks.

The machine from Honeyman rent all looks like it has a one piece grinding head.  That type will handle rougher surfaces and still get you a smooth final product.  I doubt you would need a second disc for it, those diamonds last a loooooonngg time. You may only wear a few thousandths off it in a garage size job.

(I used to own a tool rental business AND a redi-mix concrete company)

Given its a garage, I'd look for a floor grinder you can hook water up to so you can wet grind.

I did the entry hall and living/dining area in my old house after doing a bunch of patching work (removal of old tiles pulled a bunch of the concrete surface with it) so I had a smooth substrate to lay vinyl plank on. It came up really nice, would have painted well, but OMG the dust!! Being inside a house, I decided against a wet grind and even though I used a dust collector vacuum, the dust was horrific. 

Also bear in mind concrete dust contains silica, and you want to minimize your exposure to that, silicosis is not something you want

I *think* that grinder has a wet-grind attachment, but I'm going to check and make sure.

In reply to daeman :

I actually just ordered a full face mask with P100 filters.

In reply to jgrewe :

I only need an 80-grit grind on the floor. Think the disk will be enough? By the sound of it, I'll just need to make sure it's that grit of cutting wheel and pre-grind with my hand grinder.

I'm guessing it will be OK. They should be able to tell you what grit those tabs are. The coating people are trying to keep you from putting their product over a broom finish and not liking the results.

Wear a good mask, silica dust is nasty.  https://www.cdc.gov/niosh/topics/silica/grinders.html

I did the garage wet.  Much more manageable than using the turbo cup, dry.