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Fixing Bow in MDF

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Forum topic by jtrz posted 09-14-2018 03:09 PM 373 views 0 times favorited 17 replies Add to Favorites Watch
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jtrz

159 posts in 1293 days


09-14-2018 03:09 PM

About 6 months ago or more I glued together two pieces of 3/4” MDF sized roughly 2’ x 4? I was hoping to use it as a nice flat bench top. The glue-up was a nightmare because I didn’t properly have all of my clamps out and ready and just didn’t plan it well. I eventually just screwed it together in a lot of places, especially in the middle.

The biggest blunder was that I left it to dry (can’t remember if it was sitting on two saw horses or something else) with the two ends (the 2’ edges) unsupported. Those ends each had a few clamps and I had some 2×4’s clamped across as well which I threw some tools and tool boxes on for more pressure. I was so pissed off at myself for not planning ahead that I didn’t think of the problem I was creating and just left it to sit for most of the day.

So, obviously, it has a bow in it now. I’ve been using it as a work surface by just putting it on some sawhorses and have flipped it over to try and reverse the bow. I had my miter saw sitting on it for a long time as well. It has helped a little but not enough. I think it’s the fact that the glue dried while it was bowed that is the problem.

I don’t necessarily need it dead flat. Relatively flat is fine. I am thinking either I try gluing another piece of 3/4” on and clamping it correctly. Or creating a 2×4 frame to screw it to. I don’t have a thickness planer, just a benchtop jointer (should have bought the planer first) and hand planes which I am still learning how to use. I haven’t mastered truing up stock yet by hand.

I only have a contractor table saw and it really isn’t accessible at the moment because I am moving my “shop” at the moment.

So what would you do? Do the 2×4’s I attach it to need to be square and true like I am thinking or could I just throw some that are close to straight together? Should I assemble the 2×4 frame first or attach members 1 by 1?It’s $25-30 in materials so I’d like to get some use out of it. It’s definitely heavy and strong and I am essentially starting from scratch building a shop so it could be put to use in a number of situations.

Any help or ideas for it would be greatly appreciated? Thanks

-- Jeff | Louisville, Ky


17 replies so far

View Lazyman's profile

Lazyman

2397 posts in 1507 days


#1 posted 09-14-2018 03:24 PM

Depending upon how big the bow is I would probably cut my losses and start over. You’ve basically made a spring that will be a continual nuisance if you try to force it straight. If you want to salvage the material, you may be able to cut it in half perpendicular to the bow for use in future projects where the a lesser bow is not a problem.

Personally, I would not glue the 2 piece together. I would simply drive screws from the underside and make sure they are well supported by the base once installed, though 2’x4’ won’t need much support.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View John Smith's profile

John Smith

1312 posts in 283 days


#2 posted 09-14-2018 03:34 PM

I have a very flat spot on my concrete slab floor that I use to
glue up panels. . . . a few screws so it doesn’t slip-n-slide,
a layer of newspaper around the edges so the panel doesn’t stick to the floor
and as much weight on it as I feel appropriate. bags of sand, cement, cement blocks,
5 gallon buckets of water, etc etc. this will greatly reduce any “deformations” in the panel.

.

-- some people are like a Slinky - - - pretty much good for nothing. But still make you smile when you push them down a flight of stairs.

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jtrz

159 posts in 1293 days


#3 posted 09-14-2018 03:54 PM



I have a very flat spot on my concrete slab floor that I use to
glue up panels. . . . a few screws so it doesn t slip-n-slide,
a layer of newspaper around the edges so the panel doesn t stick to the floor
and as much weight on it as I feel appropriate. bags of sand, cement, cement blocks,
5 gallon buckets of water, etc etc. this will greatly reduce any “deformations” in the panel.

.

- John Smith

Thanks John. I am fan of that method and would have used it except I have been in a basement that I have yet to find a decently flat spot, at least in the corner of the basement that I can use. Problem though is that this is post glue-up. Unless you think laminating another flat sheet good on it in this fashion would help straighten her out?

-- Jeff | Louisville, Ky

View diverlloyd's profile

diverlloyd

3023 posts in 1977 days


#4 posted 09-14-2018 03:56 PM

Do you have a router? If so you can make a sled to flatten the surface.

View LittleShaver's profile

LittleShaver

365 posts in 739 days


#5 posted 09-14-2018 04:09 PM

Cut your losses and start over with fresh stock. Consider it cheap tuition in the school of glue-ups.

-- Sawdust Maker

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AlaskaGuy

4369 posts in 2429 days


#6 posted 09-14-2018 04:43 PM

I’m with LittleShaver start over.

This try this. Cut one piece to exact size and the other piece a bit oversized. Take one piece and drill and counter sink a bunch of holes in it. Glue and screw the two pieces together working from the middle out. The side with the screws will be the bottom of your top. After the glue is dry you have the choice to leave the screws, fill/putty over the top of the screws, or you can pull all the screws out and fill/putty the hole or not. When you screw it together work on a flat surface. The screw with replace the clamp that don’t work so well in this application. I done this may times when only one side shows.

I have also done the above where both side show. Cover the screws or holes with Spackle and paint or lay you finish material over the top.

If by chance your new material has any bow to it put the bows so they will pull against each other to flatten it out.

When you done take a router and flush trim bit and even up the two pieces to the same size.

What Lazyman said (no glue) works well too if you can leave all the screws in it.

-- Alaskan's for Global warming!

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AlaskaGuy

4369 posts in 2429 days


#7 posted 09-14-2018 04:45 PM



Do you have a router? If so you can make a sled to flatten the surface.

- diverlloyd

That’ll get the fine dust flying.

-- Alaskan's for Global warming!

View John Smith's profile

John Smith

1312 posts in 283 days


#8 posted 09-14-2018 04:45 PM

as said – if you can’t find a flat spot to reverse the damage,
cut your losses and start over.
and – cut it in half and lay it flat in a corner with weight on it.
you may have a need for a 2×2 piece in the future.

-- some people are like a Slinky - - - pretty much good for nothing. But still make you smile when you push them down a flight of stairs.

View jtrz's profile

jtrz

159 posts in 1293 days


#9 posted 09-14-2018 05:44 PM

I am moving shop into a garage so there may be a flat spot in there. Heard of people using routers for situations like these but I like to keep my MDF routing tasks as small as possible. My mistake was gluing it instead of just screwing it.

It’s not so bad that it can’t function as a worktable of some kind, maybe a tool restoration station that I can get filthy with grease and rust and metal particulates. A pen placed on top of it may roll a bit but usually doesn’t make it all that far.

But two 2×2 tops might work well for some benchtop tools. I feel like all of my benchtop power tools are a bit too wide for 2’ but I may be able to make something work.

If I think of some genius solution I’ll let you all know. Thanks

-- Jeff | Louisville, Ky

View jbay's profile

jbay

2602 posts in 1019 days


#10 posted 09-14-2018 06:26 PM

My mistake was gluing it instead of just screwing it.

No your mistake was not keeping it flat while it was glued. Nothing wrong with gluing it.

If your going to use the top for a bench I would assume that the bench is going to have some kind of frame work that you would be able to attach your top to and be able to take out the bow when you attach it.

I would keep it and use it for your bench top.
(Also depends on how bowed it is, but 1/4” dip should pull out easily enough)

View diverlloyd's profile

diverlloyd

3023 posts in 1977 days


#11 posted 09-14-2018 07:21 PM


Do you have a router? If so you can make a sled to flatten the surface.

- diverlloyd
That ll get the fine dust flying.

- AlaskaGuy


Yes do it outside with a respirator on. Then it could be a router sled table for all of his flattening needs.
Op how bad is the bow in the project.

View lumbering_on's profile

lumbering_on

76 posts in 610 days


#12 posted 09-14-2018 07:51 PM



Cut your losses and start over with fresh stock. Consider it cheap tuition in the school of glue-ups.

- LittleShaver

+1

Even here in Canada, MDF a 4’X8” sheet of 5/8 is only about $33 at Home Depot. It’s far cheaper than a lot of mistakes I’ve made.

View Bill_Steele's profile

Bill_Steele

407 posts in 1852 days


#13 posted 09-14-2018 08:28 PM

I agree with others—cut your losses. MDF is relatively cheap—I would start over. Like others have said, find a flat surface to work on and support the MDF while the glue dries.

You don’t need clamps for this. Do exactly what AlaskaGuy outlined. I would use screws to hold the pieces together (pre-drill and countersink since this is MDF and you don’t want it to split).

When you pre-drill, make sure that the hole in the bottom piece is large enough that the screw threads DO NOT bite into that piece. You want the screw to pull the bottom piece into the top piece. You need 2 different sized drill bits or maybe a tapered drill bit (e.g. Fuller). If I were using a #8 screw I think I use a 13/64” bit for the clearance hole.

View WoodenDreams's profile

WoodenDreams

66 posts in 31 days


#14 posted 09-15-2018 07:50 AM

If your only using Saw Horses with your board on top, try adding another Saw Horse in the middle, screw the top to it and add weight to the horse You didn’t say if you had 2×4 runners on your horses to screw the top to the 2×4’s. Screws may pull your MDF Top flat to the 2×4’. The cost of two or three 2×4’s the length of your top is cheap. All my work benches I went with the cheap method. Built the frames and put 2×4 cross runners across for a lower shelf, and 2×4 cross runners where the table top sets on. Then I screwed a single sheet of 3/4” premium MDF board to the top of my bench frame. My center work table is 4’x4’, my outer work benches are only 2’ wide. Placing cross runners where the equipment goes gives the support for weight. Holding up 2 years now and still straight. I can always add or screw on another MDF board over it later. My gluing table is just setting on 2 Saw Horses, and is a single sheet of 2”x24”x8’ laminated MDF board. been straight now for a year.

View jtrz's profile

jtrz

159 posts in 1293 days


#15 posted 09-19-2018 04:20 PM

Sorry for the delay in getting back on here.


My mistake was gluing it instead of just screwing it.

No your mistake was not keeping it flat while it was glued. Nothing wrong with gluing it.

If your going to use the top for a bench I would assume that the bench is going to have some kind of frame work that you would be able to attach your top to and be able to take out the bow when you attach it.

I would keep it and use it for your bench top.
(Also depends on how bowed it is, but 1/4” dip should pull out easily enough)

- jbay

Of course, not keeping it flat was the major mistake but I just didn’t have a large enough flat surface to use.
It was supposed to be the top of a roiling workbench or possibly a bench top tool cart that would have a frame.

However, that was a while ago and since then I have stopped using any power tools that are especially loud and ones that create a lot of dust because I don’t want to piss off my buddy who owns and lives in the house I am living in. So, no jointer, no router and no table saw. I really only use a circular saw, miter saw, and jigsaw inside. So I have started learning more traditional techniques to keep the noise down. Now I would like this to be the top of a small woodworking bench that I can put dog holes in and attach a vise too.

I checked the status of the MDF last night and it isn’t as bad as it used to be. Along its 4 ft length it has maybe a 1/4” dip and along the 2 ft width there is a very slight concave bow. So small that I would need to break out my feeler gauge to get a measurement. So a level frame could definitely take it out. I was thinking that a simple torsion type frame with 3 or 4 2×4’s running along the entire length and then a row or two of 2×4’s between them to create a grid. But even that might be over kill.

Could I get away with using 2×4’s that haven’t been jointed and planed for the frame? Without my table saw it will be hard to get nice straight plywood members for the frame and without my jointer getting the 2×4’s squared up will be difficult. My hand planing skills are fairly limited when it comes to truing stock.

Any suggestions on the frame or anything else would be super appreciated. Thanks

-- Jeff | Louisville, Ky

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