Can I use MDF outside?

[Fotoman]

I need to replace the hatch for my flybridge. I was thinking of using the usual plywood and varnish to get a classic wood look. Bu then started to think about doing it in MDF and paint it with Brightside to get a more modern look. Will MDF stay nice when left to the elements? I would seal it with a few coats of diluted resin, then prime and paint.

 

[Daddyo]

I would and have used MDO same stuff but made for exterior use ie: signs, trim work etc. I built my style bars out of it.

 

[Moonstruck]

Old Stone wrote:
Willy - STOP! Don't do it! MDF is only used inside. MDO is what sign painters use and is specifically made for exterior use. The MDF, no matter what you do to it, will fail.

*True.* MDO is medium density overlay that is over exterior plywood.* MDF is medium density fiber board.* When wet it will swell up and come apart like a cheap suit.* Do not use it outside.* For my money it has no place on a boat as it does not need to be*subjected to high ambient humidity.* Marine plywood is expensive, but they don't call it marine for nothing.* I don't think MDO is really suitable for very long haul exterior use on a boat.* You might encapsulate plywood in fiberglass.* Then make 100% certain that no moisture can enter.* I would probably use a sythetic coring material.

 

[Sailor of Fortune]

Good MDO is much better than some of the inferior grades of "Marine" plywood. The glue is the same and it often has less voids (visible on cuts) than marine.
Everthing said about MDF is correct - no place for it on a boat. If in doubt, do the "Down east scientific plywood test as used and perfected by none other than Harold "Dynamite" Payson of Thomaston.
You take a couple of off cuts and boil them (several times) and then put them outside for a couple of months to weather. Repeat several times, if you see delamination don't use. He was an ICON in the boatbuilding world who recently passed. As he once said"that plywood is tougher than chewing boiled owls!"
He used an unfinished sheet of plywood in his NEW trucks that would outlast the body. He said it spoke more to the quality of plywood than U,S built pickup trucks.
MDO is dependable aboard our small ships.
My 18' Jim Michilak designed AF4 made out of MDO and epoxy on the seams only. with care, it will outlive me.

 

[Tony B]

There is a form of MDF called Extira.
It is used a lot in making exterior signs. Like any MDF, I don't think I would use it for any structural application.

If it will suit your purposes, MDF has a lot to offer.

Tony B

 

[Daddyo]

Extira is a brand name for MDO. When I used the MDO just for good measure I applied two coats of epoxy before priming. Overkill, yes!

 

[Moonstruck]

Daddyo wrote:
Extira is a brand name for MDO. When I used the MDO just for good measure I applied two coats of epoxy before priming. Overkill, yes!

*Nope, not overkill.* That is what I would have done----especially at the ends.* Fashion plates are structural for roof support.* The strength of wood was needed, but as you know if moisture can get in wood it will.* Looks like a great job.

 

[Sailor of Fortune]

Carl
she will do 20 mph with 15hp but these boats have all the usual shortcomings with flat bottom power sharpies. They pound in a chop and are wet in windy conditions. She will be perfect in the St Johns river and ICW. I have put it on the back burner because I am moving to Orange Park, Fl from St Aug.

 

[Tony B]

MDO is DEFINITELY NOT, the same product as Extira.
MDO is a structural (of sorts) plywood with an overlay on both side with a product similar to Extira. It is the overlay ONLY. The rest is exterior plywood.
Extira is a composite material that is not structurally sound but is extremely weather resistant. It looks and machines like MDF but the chemical content is different.
I would consider building a dinghy with MDO and definitely not with Extira.

 

[Moonstruck]

In my past life I was a territory manager for both Georgia- Pacific Corp. and Boise-Cascade Corporation.* They were two of the largest manufactures of wood products in the US.* I was also National Sales Mgr. for Bradley Plywood Corp.* They were manufacturers of high end architectural panels.* Over those 15 years I did a lot of trouble shooting claims.* What would be found is many times the wrong product was used or instructions for proper installation ignored.

There have been many large scale class action suits against wood manufacturers.* You can probably guess what they were mostly about-----wood composition products.* Masonite and Louisiana-Pacific sidings mostly. G-P flirted with composition backed vinyl faced panelling.* It was a disaster.* Manufactures have tried for years to make products from chips and saw dust.

Today there are many "engineered" products to use in construction.* Some structural.* You just have to be careful to use the right product carefully. Two sources for info on products are the American Plywood Assoc. and the Southern Yellow Pine Inspection Bureau.*

Check the product, and follow instructions.* Composition board usually have no place on a boat.* The following picture of Miss OSB may be an exception.* The chips were bonded with West System Epoxy.

 

[Fotoman]

Ok, so no MDF on a boat. Got it. I think I'll stick with plywood since MDO is not readily available here. Thanks for all your input.

 

[davechili]

If you don't care for wood, consider Azec or similar pvc products. Not inexpensive, but no rot and easy to work.