Author Topic: diy countertop bowing  (Read 3362 times)

nereo

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diy countertop bowing
« on: March 12, 2016, 01:45:34 PM »
Here's my problem

I'm building a counter-top and it unexpectedly started bowing on me.  Top layer is 3/4" MDF (yes, that's probably part of the problem) with cross-2" thick bracing strips of 3/4" plywood underneath (to make it 1.5" thick) and 1/4" pine around the edges.  Ultimately the surface will be coated in epoxy resin which was the reason I used MDF and not 3/4" stock to begin with. 

It's roughly 84" x 15".  Now it has about a 0.5" bow measured in the center. I weighted it down with bricks for a week which may have lessened the bowing, but it's still present.
a) any idea how to fix this now?
b) wtf happened?  I thought using the plywood bracing strips would eliminate this problem.



zolotiyeruki

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Re: diy countertop bowing
« Reply #1 on: March 12, 2016, 05:07:42 PM »
It looks like the bowing is in the direction so that the middle of the countertop is higher than the ends--is that right?  If so, I wouldn't sweat it too much.  When you install it, put some heavy stuff on the middle to force it flat, and drive screws in from underneath to keep it that way.

A 7' span is an awful long distance for wood to keep itself straight.  Personally, I probably would have used MDF for the braces underneath, so any expansion/contraction would be the same for both layers.  Thickness is probably more important than the actual material used for the bracing for this kind of geometry.

What kind of epoxy are you using to coat it?

nereo

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Re: diy countertop bowing
« Reply #2 on: March 12, 2016, 06:49:59 PM »
Thanks zolotiyeruki - yeah, the only "solution" that I had come up with was to do basically what you described... weight it and then screw it in from underneath. And yes, the bowing is in the middle, so it's about 0.5" (unweighted) in the center relative to the two sides. 
Never considered using MDF for the braces as well so that the materials were the same.  Oops... rookie mistake.

The epoxy is a two-part clear epoxy resin.  I'm embedding stuff into the MDF and then the actual counter-top surface will be the epoxy.  I tried first with a small end table that worked brilliantly, but you're right... 7' is a long stretch to keep wood straight. 
Live and learn... thanks for your feedback.

paddedhat

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Re: diy countertop bowing
« Reply #3 on: March 13, 2016, 03:21:54 PM »
I have subbed out a ton of custom made "Formica" tops over the years, as a homebuilder. Many tops were significantly larger than yours, and fabricated on site. They were always 3/4 MDF, with another layer of 3/4 MDF strips attached to the bottom, for stiffeners and to create the appearance of a "thicker" top. The stiffeners are attached with yellow glue and 1/4" crown air staples. I have never had issues with tops bowing or sagging.

 Before you go any further I would place the top on a flat surface, block the ends up on some 3/4" scraps and place some weight in the center to straighten the bow. Leave I like this, at least overnight. I once had a painter who was some kind of sorcerer, when it came to getting bows out of doors. He would put them on horses, shim at different locations, pile weight here and there, and leave them in the sun for a while. It usually worked, somehow, LOL.

worms

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Re: diy countertop bowing
« Reply #4 on: March 14, 2016, 01:02:03 AM »
I'm not sure the 3/4"inch ply is doing much, since it might be much more bendy laid flat than the mdf itself would be.

nereo

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Re: diy countertop bowing
« Reply #5 on: March 14, 2016, 08:06:16 AM »
I'm not sure the 3/4"inch ply is doing much, since it might be much more bendy laid flat than the mdf itself would be.
The 3/4" ply was added to make the countertop 'thicker' as well as to act as 'stiffeners' (without it the span would flex on its own under heavy weights).  From this thread I've learned that I should ahve just used MDF for all of it, but I had some 3/4" ply strips available at the time.  As I said... live and learn.

Quote
Before you go any further I would place the top on a flat surface, block the ends up on some 3/4" scraps and place some weight in the center to straighten the bow. Leave I like this, at least overnight. I once had a painter who was some kind of sorcerer, when it came to getting bows out of doors. He would put them on horses, shim at different locations, pile weight here and there, and leave them in the sun for a while. It usually worked, somehow, LOL.

Thanks paddedhat - I'll give this a try.  I had pondering whether to try to 'reverse bow' the top doing basically what you mentioned (propping up the two ends slightly, weighting it down and keeping it there overnight).
I did use crown staples and wood (yellow) glue to fix the plywood stiffener strips to the MDF... so those aren't coming off.

lthenderson

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Re: diy countertop bowing
« Reply #6 on: March 14, 2016, 08:23:56 AM »
I work a lot with plywood and it bows pretty easily especially when glue is involved. I would just screw it to the cabinets from underneath and it should be fine. Otherwise, I have done the weighting it down and leaving it sit for awhile thing but that takes time and sometimes I have over corrected.