Warping in MDF/HDF

[dlu]

I'm trying to figure out why HDF (and I assume MDF too) sometimes warp, and how to straighten a frame and panel HDF door.

I helped a friend build a set of three Shaker style HDF doors using HDF. The doors are about 16" x 40" with 2.5" rails and stiles.
Of the set two are nice and flat, but one is warped so that the upper corner on the non-hinge side is about a 1/4" out when the door is closed.

I'd appreciate any idea for how to fix the warped door (if that's possible) and also on how to avoid problems with warping in the future.

 

[mino]

HDF "holds more together" while MDF does not ... this actually makes HDF more prone to warping.

Most of the time it is about paint - a strong coat of paint can create enough tension for to board to warp.

Second is when flooring, or other one-side-treated HDF is misused for furniture.

Lastly, heat, eg. from a stove next to it, can affect the paint only on the outer side making the tension be only on one side, having same effect as assymetrical paint.

In all cases, you want MDF for shape stability, not HDF in my experience. All the HDF flooring boards I saw, for example, warped after some time if they were not in the floor ..

 

[Peter Kelly]

If you're making 5-piece type doors I'd spend the extra money for a moisture-resistant MDF like Roseburg Medex. If you've got a moisture meter, you might want to also check your panels before fabricating anything. MDF shouldn't be over 10%.

 

[dlu]

Well, I now have some great ideas for my next set of doors...

If my memory serves, the warped door was warped from the start (before being painted). All of the rails and stiles are from the same sheet of HDF -- though I have no idea where in the sheet the strips for this door came from. Is it possible/likely that when you break down a sheet of HDF that there are internal stresses that can be released and cause the pieces you're cutting off to distort?

Finally, any suggestions for how to fix the warped door? Or, do I just make a new door?

 

[Peter Kelly]

It's possible that the unfinished panel you'd used was stored in such a way that it absorbed a lot of moisture on one side which caused warping and this may have been magnified by cutting it into smaller pieces. I've also seen others glue MDF rails and stiles to an MDF center panel but haven't tried this for fear the end product would warp....

No real way of saving a bent door, just simpler to re-make it. 

 

[Crazyraceguy]

I built all of the doors in my daughter's entire kitchen from MDF. 3/4" for the rails/styles and 1/4" for the panels. The joinery was done with a Freud bit set and a single 6mm Domino in each joint, for a little extra. They were all sealed/primed with BIN, a shellac-based product, and painted with a white pre-catalyzed lacquer. No water-based product touched them except for the glue. They have been in place for 2 and 1/2 years with no issues.
I have somewhat settled on this because of warping in the past. I have tried Poplar and Maple for rails and styles, always throwing out at least a part or two. They move/twist when cut from the bigger stick.
There is nothing you can do (without making them thinner) to fix it. One warped piece, rail or style, will warp the whole door. Make sure they are flat when clamped, a straight-edge across the joints, and not twisted as they lay drying.
There are a couple of "work arounds" if the door is not too badly warped. Positive latches, like ball catches, might provide enough tension to hold it shut, but those are a compromise to normal function.
Magnets can also do it, but you'll notice them too, in regular use.
If those don't work, or you can't live with  them, you are looking at re-making the offender.
If the MDF is warped from improper storage, I wouldn't use it. I have never had it move from cutting, like hardwood can, but if it starts bent, it will probably stay that way.

 

[Packard]

If you are using a water-based paint, then paint both sides.

Before the pandemic, the flash surfaces of MDF were calendared so hard that they were shiny. Apparently, running the sheets through the rollers that hard left the flat surfaces immune from “raising the grain”.

But recently, the flat surfaces are not nearly as shiny, and will raise the grain with water based paint. On thinner panels, I would imagine that would induce warp, unless you painted the reverse side right away.

An easy fix is to use a shellac based primer.  B-I-N is a pigmented shellac.  It is for interior applications, though spot priming knots is OK outdoors.

Sealcoat is a non-pigmented clear, that is an effective primer for indoors applications. It contains less shellac that the same amount of BIN.

You cannot use just any shellac as a primer.  It has to be dewaxed.

The saying is, “Shellac sticks to anything, and anything sticks to shellac.”  That is largely true, but when spraying Benjamin Moore’s Advance over Sealcoat, I’ve experienced crazing on the surface.  I have not heard this from anyone else, but always test before using.

So, if you apply Sealcoat prior to painting, it will be less likely to warp.  Sealcoat takes about 20 minutes to dry and is ready for recoating. It brushes on easily and the smell goes away quickly.  But provide adequate ventilation when working with it.

 

[Crazyraceguy]

[member=74278]Packard[/member] I always heard it as "Wax sticks to everything and nothing sticks to wax" but I guess in this case, it the same thing  [big grin]

 

[Packard]

Apparently, it is more often:  Shellac sticks to everything and everything sticks to shellac.

Plus, shellac is edible (renamed “confectionary glazing”.  Every M & M you ever ate was covered with that stuff.
https://www.google.com/search?q=shellac+sticks+to+everything+and+every+thing+will+stick+to+shellac&client=firefox-b-1-m&sca_esv=6d9a7514d209d2c4&sxsrf=AM9HkKmpAFZr24OE6CTZqQs_-SIcmehcqg%3A1704054911611&ei=f9CRZdzRJI-ZwbkPx8OIwAw&ved=0ahUKEwicjbydw7qDAxWPTDABHcchAsgQ4dUDCBA&uact=5&oq=shellac+sticks+to+everything+and+every+thing+will+stick+to+shellac&gs_lp=Egxnd3Mtd2l6LXNlcnAiQnNoZWxsYWMgc3RpY2tzIHRvIGV2ZXJ5dGhpbmcgYW5kIGV2ZXJ5IHRoaW5nIHdpbGwgc3RpY2sgdG8gc2hlbGxhYzIIEAAYgAQYogQyCBAAGIAEGKIEMggQABiABBiiBDIIEAAYgAQYogRIraMBUABY354BcAB4AJABAJgBcKAB2QaqAQM5LjG4AQPIAQD4AQHCAggQABiJBRiiBOIDBBgAIEGIBgE&sclient=gws-wiz-serp#ip=1

 

[Packard]

I would note that MDO seems to be replaced in many (perhaps most) instances with expanded PVC sheets. I cannot imagine that these would have sufficient structure for a table top though.
https://www.grimco.com/Catalog/Category/Substrates/PVCSubstrates?utm_source=adwords&utm_campaign=PVC+Sheets&utm_medium=ppc&utm_term=foamed%20pvc%20sheet&hsa_src=g&hsa_kw=foamed%20pvc%20sheet&hsa_tgt=kwd-580654543404&hsa_cam=1614463141&hsa_ad=670061188033&hsa_net=adwords&hsa_grp=59141955057&hsa_ver=3&hsa_mt=p&hsa_acc=6995058817&gad_source=1&gclid=Cj0KCQiAv8SsBhC7ARIsALIkVT1h7mJyyQ_pVWMC_HM6ZRs5j85l-AOKx9q3lbuj1f-8Azy5UsCmbhYaAnJpEALw_wcB

 

[woodbutcherbower]

Every M & M you ever ate was covered with that stuff.

Have you ever seen the legendary 1990’s Van Halen tour rider which specifies a dressing-room large bowl of M&M’s (ABSOLUTELY NO BROWN ONES)? The band’s tour manager put that clause into the rider contract not because the band guys had anything against brown - but he did it make sure that the show promoter had actually read the contract.

I wonder how an ABSOLUTELY NO SHELLAC clause would have been complied with?