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I recently had a stone countertop installed with a mitered edge. For support, the installer put plywood under the stone. However, because the slab wasn't flat on the underside (to my surprise) - the plywood is sticking out in places about 3/16".

Additionally, the plywood he used wasn't big enough so it has a couple of pieces screwed into it to make it "flat" (obviously provides little support).

Sidenote: The slab is also supported by metal brackets, not just plywood.

I want to make the underside of the countertop smooth and need help with finding the best way to do that.

I was thinking I can remove the screws then use a planner to shave down the plywood, then use Bondo wood filler to make it flat, then sand and paint it.

Or I could use a belt sander instead of a planner.

Any other, easier, ways to accomplish this?

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    I would be talking to the installer and/or the maker of the counter top, since it is new and should still under warranty.
    – crip659
    Jun 23, 2022 at 13:34
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    That's a poor installation job for sure! I'm assuming you paid big money for this countertop and installation. I'd be contacting the company you purchased from and express your dissatisfaction with the installation job.
    – jwh20
    Jun 23, 2022 at 13:46
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    What is your goal in making it "flat"? Are you concerned that other people will see it sticking out? Are you concerned that you'll snag clothing on the rough wood underneath? Also, no, you can't use a planer on plywood, you'll end up destroying it, especially if you're trying to remove 3/16" of an inch.
    – FreeMan
    Jun 23, 2022 at 15:30
  • Assuming the installer isn’t going to fix this, your best bet is to box this in with thin plywood and decorative edging. Jun 23, 2022 at 17:04
  • Belt sander will take the excess off very quickly but will make a huge mess so consider that...
    – Kyle B
    Jun 24, 2022 at 5:16

2 Answers 2

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If you try to remove this material using a belt sander or a power planer you will find it makes a tremendous mess and will be difficult to get it smooth. Additionally, you weaken the subcounter and risk hitting and cracking the slab.

Assuming the installer will not address it, I would mask the stone, caulk the gap, and try painting it flat black. This will make it look more intentional and help hide it from casual glances. This is simple and if you still cannot live with it after painting then you can try more difficult mechanical removal.

Your install is not correct, but it's not easy to correct. The subcounter is the wrong thickness and the fabricator easily could have measured in advance. It's nothing to do with the underside of the slab not being flat. If that were a problem then the fabricator could have cut out the subcounter material directly beneath the bulging part. Additionally the subcounter isn't even cut straight.

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it has a couple of pieces screwed into it to make it "flat" (obviously provides little support).

Take that first row out since they do nothing. I'm assuming the row that abuts the cab cannot be removed, so you'll have to bevel its edge so you don't clonk your knees on it, in-situ. The first row you just don't put back. It's not going to be flat under there, but anything is better than plywood proud of the stone giving you paper cuts on your thighs.

If it's a pour and that first row is trapped in there, and they don't just fall out when you remove the screws... then have fun when your planer hits that stone while you're upside down. Or you need a stone polishing kit after you accidentally hit it with a RO or belt sander.

Maybe just ease that edge off... very carefully.

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  • ... how good are you with a Fein saw?
    – Mazura
    Jun 24, 2022 at 1:57

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