Dimensional stability: Plywood vs MDF

In my experience, yes. MDF has little or no grain, which is what gives the problems.

When I was at the shipyard we work quite a bit with MDF. (making items like furniture with a CNC router)

Problem with the MDF, the inner core of the MDF is looser (less dense) that the outer skin, Plus the laminated MDF as well as just MDF could change thickness, because the dimensional stability of the material, such as thickness can change with the humidity.

But if it did not get wet, it was'nt bad, as far as the the thickness change due to the weather it was consistent, but that does'nt sound like your problem.

We used vacuum to hold it in place, but the MDF was pretty flat.

Just be cqreful with getting too light of density, like I mentioned earlier. too light of density, nothing is holding it together and it can fall apart after to outside surface is removed such as a dado, rabbit or counter bore is used.

And not having grain can also work for you as well as against you.

We did have better initial proformance with MDF, but the plywood was more durable.

We used MDF, baecause it met the military requirements.

phoenix911

I used to build light-tight testing booths for doctor's offices and found mdf to work very well. It never warped, was easy to work, and came in consistent thicknesses.

I am very much surprised that your plywood warped. I have never seen that. But you get different grades of ply. Over here we have what we call "marine ply", which I believe is similar in performance to what you get from (baltic?) birch ply; this is generally considered to be of better quality. I don't know too much about plywoods, but I suggest you investigate the better grades. At my work they recently cleaned up an old store. Against a wall was a few plywood sheets; they were the sides from some packing crates. Not one of them was bent or warped, even though they had been leaning against the wall for goodness knows how long.

The best way to stabilise wood products dimensionally is to seal it. And some woods move more than others. I suspect that hardwoods move less.

GA from me.

I feel also that the ply you used had a quality that was very questionable.

Marine ply has a standard (from memory only, maybe someone else can substantiate this also) that includes not de-laminating after 24 hours in boiling water.......this stuff is so stable it is really unbelievable.....in 3/4", it is almost like steel in stiffness!!!!! It should not really need sealing if used indoors, only for good looks maybe......I will be using it as a floor for my Balcony next year.

MDF is great stuff, but sucks up moisture like a sponge and although stable with no stress, once stress is applied it takes a "set" quite quickly.

The biggest problem with marine ply in Germany is that nobody wants to cut it for you on their highly accurate saws, it blunts the blades too quickly!!! I need high repeatable accuracy that no "Home" system will achieve!!! So I use plastic coated MDF, stable but it has nowhere near the qualities of a good marine ply......

The Advenger Class Minesweeper MCM 1 through MCM 14.

The were built from wood, wood comsites, FRP, Non-ferrous metals, and polymers.

The major cause of death on minesweepers is slivers.

Had alot of additives to those liminates. Some of the laminates and shiny speckles on it.

When I asked for MSDS sheets on the stuff, my life got tougher. Was not a good thing to ask.

But yes it was very stable.

But, those stable laminates had a price tag on it.

not only the base woods, but the internal laminates that bond/reinforce them. you get what you pay for.

Hello Laughing Jaguar:

I am very surprised the plywood you got warped.

I would always use 'marine-ply' which uses better wood, more care it taken over its composite design, and is much better that ordinary ply at taking any moister from the surroundings.

With mdf, you get a more stable board at you go up in thickness (as you also get with ply) but it will not take edge stress. An instance you cannot use it as a door. It is good for 'shapes' as long as it is supported by other mdf on other wood. But, as its name suggests it is made from fibre, actually, more like dust. I will search for the different grades of mdf and send them to you.

Given the choice I would go for marine ply and a good coat of polyurethane. Actually, do a couple of very thin coats, will will bring the 'fibre' up. Rubbing down between the first two coats but, only enough to make it smooth, with find glasspaper. Then continue to do the next two coats or more. I have done the same using 1.5" ~38mm for table tops, with no bending.

I am not sure why you are not using mechanical fixing. Whether it is ply or mdf, you will need to choose the correct adhesive which will stick whatever you want to the polyurethane, as this is what you will be sticking any other surface to.

Good luck and happy holiday...........

Hello again Laughing Jaguar, I see that you have wandered away from your profession again, "what ever it is. Never the less, I will attempt to provide you with a little wisdom about ply woods First there are asst. construction grades that are suitable for rough construction {residential framing and sheeting}, there are also better grades such as pre sanded ply form used for concrete form work. The above grades have the inner core sections running at diagonal and verticle as well as parallel to the outer plys. The more ply's the better the grade of plywood. The aforementioned plywoods generally are glued to gether with exterior quality glue.

Interior grades of plywood may be assembled much the same as exterior plywood except that the surface skins are usually a&a or a&b grade and the glue is of a grade suitable only for interior use.

Then you step up to the cabinet and furniture grades. These tipically have various grades of interior material filler, You can get different grades of partical board, coarse to very fine, a genuine plywood core or a lumber core. Then you might consider the Marine grades, still used in boat construction, and also used in air craft construction as sitka spruce plywood is almost non existant any more. Regarding these last two options you will need to choose the relationship of the direction of the interior and exterior plys.The interior plys can be laid a a 45 degree angle to the skins, are less expensive and more flexable than those installed a right angles. Plywood skins can be made from most any wood desired.

In short, "know your plywood specific needs vs performance desires and purchase accordingly. It's just good sense and saves heart aches and money.

Cheers

TMF

Well, I heard a lot about a powder as a great mdf coating, but what about lacquer or shellac?

Hi....use genuine 'Marine Ply'. It also goes by the name 'Boiling Water Proof' (BWP). These do not wrap and are very stable in shape & dimentions.

Fyi, BWP ply was invented during WWII. Most RAF aircraft was made of plywood. During the early stage of the war when the Japanese were invading Burma the body parts of aircrafts sent from England used to wrap and the ply open up in the highly hot & humid conditions. My father used to have a tough time tightening alluminium rivits all over by hand. It was after this that in the UK they developed a perfect BWP ply and all metal planes to fight the war.

AKT/Delhi

Hello ronseto:

That sign writing board sounds good stuff! Possibly the right thing for this application as, from the sound of it the board is strong even in thin sheets?

Take care and happy holiday..............

GA for Ronseto!

As you imply....

There are many grades of plywood and even more of engineered composite board material. You can't just compare "plywood to MDF" In better quality cabinets the shelves used to support book cases and wide spans for heavy dishes are never plywood. This is for warping, cupping, saggign and strength. In home construciton the engineered wood products have proven superior than wood.

The local cabinet shops that advertise "all wood" consistantly try to discredit flake board and oriented strand board as cheep and won't last. I am sure some products are builf from the wrong boards.

It takes an educated consumer to know what is available and works.

Will the use of MDF instead of plywood provide me with greater dimensional stability?

YES, I made wax pattern setters out of MDF that worked great.

Don't get it wet.

If you do use mdf instead of marine ply: consider using super glue (cyanoacrylate) to seal the cut edges before painting (mdf "sucks").

Do you get the thicker grades of Masonite? Masonite is much more durable than MDF. The thick grades are used for external signs. MDF sucks up so much moisture I'm considering using it as a seedling raising medium. If it works I'll post it.

"I'm considering using it as a seedling raising medium. If it works I'll post it."

I doubt that MDF has the porosity needed to supply moisture for developing plants. Also of concern to me would be the potential for leeching of the chemistry from the binding resin.

It's still a novel idea and worthy of a test.

L.J.

perhaps try some Marine grade plywood?

Groovycbr

Thanks to all who responded to my question.

I am convinced that the error was mine. I should not have purchased plywood paneling from Home Depot, especially given other prior experience with their lumber.

"You get what you pay for."

I've asked for a price quote from my local lumbar yard for 4' x 8' sheets of 3/4" MDO and also for Marine Grade Plywood.

Thanks

L.J.

MDO: acronym finders all have:-

Medium Density Overlay (engineered plywood)

AND, Machine Direction Orientation (polyethylene sheeting)

Depending on application a man made material might be worth considering.

How embarrassing!

I've five different projects going, all of them using cored panels of low density structural foam and fiberglass. 30 years of working with these materials and yet, it never once occurred to me to make up the panels myself using the same or similar technque!

I returned to this string to let everyone know that when the 3/4" MDO (single side) arrived, we checked and found it warped! The lumber yard is now looking for "furniture grade plywood".

In the meantime, I will explore the costs of building these panels from 1/2" thick Divinycell foam cores laminated between a sheet of fiberglass on one side and a sheet of Formica on the other!

Thanks Randall! I'm awake now!

L.J.

Single sided MDO will warp. You needed two sided MDO.