# Sealing MDF Joint



## Frenchy93 (Mar 4, 2016)

Hey all,
I boxed in a vertical support post on stairs in a house I'm doing with MDF, the box is 6(in)x4(in)x8ft high. 

What is my best bet for sealing the joints? Need something durable as occasionally people will be grabbing onto it when descending down the stairs. 

Obviously sandable and paintable. Do I just go with something like a drydex?


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## Big Johnson (Jun 2, 2017)

Frenchy93 said:


> Hey all,
> I boxed in a vertical support post on stairs in a house I'm doing with MDF, the box is 6(in)x4(in)x8ft high.
> 
> What is my best bet for sealing the joints? Need something durable as occasionally people will be grabbing onto it when descending down the stairs.
> ...


Picture? 

I like pTimbermate


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## pinwheel (Dec 7, 2009)

I use bondo in those applications. Just finished a fireplace surround, mantel & book cases with partial MDF. All joints were glued & gaps filled with bondo before painting.


I sent the builder a text in the middle of building it, "if you ever hear me say I love working with MDF, call the police, it's code for I've been kidnapped." I hate MDF


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## Frenchy93 (Mar 4, 2016)

pinwheel said:


> I use bondo in those applications. Just finished a fireplace surround, mantel & book cases with partial MDF. All joints were glued & gaps filled with bondo before painting.
> 
> 
> I sent the builder a text in the middle of building it, "if you ever hear me say I love working with MDF, call the police, it's code for I've been kidnapped." I hate MDF


Pinwheel,

You just talking like this or actual body filler?









Doesn't look like the wood filler is available in Canada. THe body filler is for sure though.


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## pinwheel (Dec 7, 2009)

Frenchy93 said:


> Pinwheel,
> 
> You just talking like this or actual body filler?



Either one will work. I sometimes use automotive glazing compound as well. The wood bondo & glazing sand a lot easier the regular bondo.


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## Big Johnson (Jun 2, 2017)

The glazing compound shrinks too much, not sure about the other bondo stuff. Some people love making extra work for yourselves.


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## Big Johnson (Jun 2, 2017)

!!!


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## pinwheel (Dec 7, 2009)

Big Johnson said:


> The glazing compound shrinks too much, not sure about the other bondo stuff. Some people love making extra work for yourselves.


Not that I've noticed. I used it on a set of painted cabinets last winter. Put eyes on them again late this summer & couldn't see any issues. I specifically looked at that because it was the first time I'd used it.


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## tang (Jan 5, 2009)

Not sure I understand. You built the box using butt joints? and want to hide the joint line? I'm envisioning MDF face nailed to a post, one piece at a time.... Tippy style....


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## Frenchy93 (Mar 4, 2016)

tang said:


> Not sure I understand. You built the box using butt joints? and want to hide the joint line? I'm envisioning MDF face nailed to a post, one piece at a time.... Tippy style....


Nope, 45's on most angles, but it runs into a couple obstructions in some places like stair stringers etc.


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## Frenchy93 (Mar 4, 2016)

Big Johnson said:


> The glazing compound shrinks too much, not sure about the other bondo stuff. Some people love making extra work for yourselves.


Hey Big Hammer, 
Thanks a lot for showing me that! Unfortunately, not accessible around me


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## 91782 (Sep 6, 2012)

Yeah I've had trim carpenters wrap with mdf and butt joints at that.

Strike all 4 sides with setting compound, sand with a drywall hand sander and paint.

Turns out slick.


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## tjbnwi (Feb 24, 2009)

Bondo will work just fine. I’ve never had it or glazing compound shrink. Maybe I’ve been lucky over the last 55 years.

Seeing as your just packing edges and seams the Timber Mate will work well. Timber Mate dries out quickly, make sure you cover the container well when finished. 

Tom


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## Frenchy93 (Mar 4, 2016)

SmallTownGuy said:


> Yeah I've had trim carpenters wrap with mdf and butt joints at that.
> 
> Strike all 4 sides with setting compound, sand with a drywall hand sander and paint.
> 
> Turns out slick.


Just drywall compound? This was my original idea but felt that may crack eventually no?

The end goal is to paint it in primer/trim white.


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## Frenchy93 (Mar 4, 2016)

Just purchased some body filler, I'll let you know how it goes.


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## 91782 (Sep 6, 2012)

Frenchy93 said:


> Just drywall compound? This was my original idea but felt that may crack eventually no?
> 
> The end goal is to paint it in primer/trim white.


Setting Joint Compound. aka Durobond.

Unless the joinery is really bad, you aren't putting anything on that is staying that will have any depth.


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## Big Johnson (Jun 2, 2017)

tjbnwi said:


> I’ve never had it or glazing compound shrink.
> 
> Tom


Bondo brand? Maybe I got a bad batch. It’s dark red too which is not good for painting over with white unless you plan to do a bunch of coats.


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## 91782 (Sep 6, 2012)

Big Johnson said:


> Bondo brand? Maybe I got a bad batch. It’s dark red too which is not good for painting over with white unless you plan to do a bunch of coats.


KILZ first, then paint


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## pinwheel (Dec 7, 2009)

SmallTownGuy said:


> Setting Joint Compound. aka Durobond.
> 
> Unless the joinery is really bad, you aren't putting anything on that is staying that will have any depth.


I wouldn't think water on MDF would be a good idea. Heck, even glue will swell MDF. I can't imagine what water in joint compound would do to it.


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## 91782 (Sep 6, 2012)

pinwheel said:


> I wouldn't think water on MDF would be a good idea. Heck, even glue will swell MDF. I can't imagine what water in joint compound would do to it.



Yes. Well it works.

The stuff Johnson posted - it works and it is also water based.. You aren't bathing the MDF in water.

If it didn't work, I wouldn't use it. And I'd report I tried it and it failed.

Gets used very frequently on new builds.


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## 91782 (Sep 6, 2012)

SmallTownGuy said:


> Yes. Well it works.
> 
> The stuff Johnson posted - it works and it is also water based.. You aren't bathing the MDF in water.
> 
> ...


Fair warning: If the MDF is subjected to motion or humidity changes, its gonna show in those joints and none of these compounds listed are going to hold up.

Most frequent "out of warranty" complaint on new builds are joints opening up after 2 years or so. All over the house doesn't matter. 

Floors, walls, ceilings, painted or stained. Stuff moves.


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## Frenchy93 (Mar 4, 2016)

So,
I got to work with the bondo last night, first batch I made I had a bit too much hardner and it dried up in about 5 minutes haha.

Besides that, it went really well, and it was good stuff to work with. I made a complete mess with it but lets see what it looks like after I take the sander to it tonight.


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