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Cope jointing MDF crown moulding?

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  • Feb 10th, 2021 1:14 pm
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[OP]
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Jul 7, 2017
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Cope jointing MDF crown moulding?

I'm start to cut crown moulding for a room (the existing frilly stuff in the house which I don't like, and was installed upside down to boot). Only matching moulding I can find is MDF (that has to come all the way from Chile), no solid wood available (looks like the mighty forests of British Columbia hasn't got the wood to make any). I'm trying cope jointing with the off cut to make a template and am finding it a little brittle. Using a hand coping saw and a Dremel but I don;t think I have the right tools for the latter.

Should I give up and go for mitre on the inside joints?
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Member
Feb 26, 2019
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Ottawa
thriftshopper wrote: I'm start to cut crown moulding for a room (the existing frilly stuff in the house which I don't like, and was installed upside down to boot). Only matching moulding I can find is MDF (that has to come all the way from Chile), no solid wood available (looks like the mighty forests of British Columbia hasn't got the wood to make any). I'm trying cope jointing with the off cut to make a template and am finding it a little brittle. Using a hand coping saw and a Dremel but I don;t think I have the right tools for the latter.

Should I give up and go for mitre on the inside joints?
If it will be painted you can just mitre it and no one will notice. MDF won’t have seasonal movement so the joint shouldn’t open up. Different story with real wood, this is where coped joints really make a difference.

That said, you don’t need a template to cope a joint. Just cut it as though you were going to do an inside mitre, then cut behind the profile that creates. Check YouTube for a few videos of it. There’s a good simple one of Tom Silva (this old
House) doing it with good straightforward instructions without the “hey YouTube” crap.
Sr. Member
Jun 26, 2019
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What's the angle of the corner and do you have a pic of the molding profile?

Do you own a respirator and an angle grinder?

Or a jigsaw?

Of late I've been doing most of my coping with an angle grinder, before that I used a coping foot on a jig saw.
[OP]
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Jul 7, 2017
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All insides are 90 degree (more or less).

No this one but very similar

Image

Have the particle masks, angle grinder (heavy, corded SOB) and jig saw but think they are too HD for this.

The MDF is very fragile and starts to come apart especially when cutting with internal angles <<90 degrees.
Cream rises to the top. So does scum.
Sr. Member
Jun 26, 2019
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thriftshopper wrote: All insides are 90 degree (more or less).

No this one but very similar

Image

Have the particle masks, angle grinder (heavy, corded SOB) and jig saw but think they are too HD for this.

The MDF is very fragile and starts to come apart especially when cutting with internal angles <<90 degrees.
If you can wield your angle grinder or really just clamp down the molding and use 2 hands, I'd buy a flap disc and go to town.

Just take your time and go slow/carefully.

Pretty simple, probably lots of youtube videos on it.

Alternative would be to buy a coping foot for your jigsaw, but seeing as this is a one off I wouldn't recommend.
[OP]
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SubjectivelyObjective wrote: If you can wield your angle grinder or really just clamp down the molding and use 2 hands, I'd buy a flap disc and go to town.
What grit should I get? Was looking at it.

Also have this old, heavy Craftsmen rotary that is like a big Dremel but not sure if I can get the bit for it.
Just take your time and go slow/carefully.
That's the whole thing. That heavy corded SOB I have is heavy, lots of torque.

Videos I've seen are with battery-powered grinders.
Alternative would be to buy a coping foot for your jigsaw, but seeing as this is a one off I wouldn't recommend.
15 corners to cut for. Have a corded Dewalt and have to see if there is such a beast an how much.
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Sep 7, 2015
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Ottawa, ON
dottawat wrote: If it will be painted you can just mitre it and no one will notice. MDF won’t have seasonal movement so the joint shouldn’t open up. Different story with real wood, this is where coped joints really make a difference.
I'll have to respectfully disagree. They do open up. Specially inside corners. A bit of regular wood glue at every joint. Takes 2 seconds tops.
Sr. Member
Jun 26, 2019
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thriftshopper wrote: What grit should I get? Was looking at it.

Also have this old, heavy Craftsmen rotary that is like a big Dremel but not sure if I can get the bit for it.



That's the whole thing. That heavy corded SOB I have is heavy, lots of torque.

Videos I've seen are with battery-powered grinders.



15 corners to cut for. Have a corded Dewalt and have to see if there is such a beast an how much.
I usually use 40 or 60 grit on my cordless dewalt. If you have lots of power and aren't going to go fast probably go with 60.

For lots of cutting I'll saw off more for less dust.

Check out collins coping foot, see if it fits your model.
Deal Addict
Nov 2, 2005
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I've done lots of mdf crown using a hand coping saw followed by a dremel with carbide burrs. And yes, mdf absolutely will move seasonally.
Member
Feb 26, 2019
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I should have been more clear. MDF does not move seasonally in the *same way* as wood. Yes, it can swell and shrink with changes in humidity, but it does not have the same grain structure as wood that creates the more dramatic movements. MDF mitres,, when properly glued, hold together very well, especially in paint grade situations, when you can cheat with some caulking. But, yes, unglued mitre will very likely open up, even with MDF. By contrast no glue in the world will keep a wood joint together if it wants to move.

But, as said here, go for the coping joint - it’s not that hard after some practice. Wear a mask if your grinding/sanding MDF. That dust is nasty stuff.
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Dec 25, 2007
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Maybe it's just my lack of skill but I was never able to cope MDF cleanly. It always chipped.
[OP]
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Jul 7, 2017
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dottawat wrote: I should have been more clear. MDF does not move seasonally in the *same way* as wood. Yes, it can swell and shrink with changes in humidity, but it does not have the same grain structure as wood that creates the more dramatic movements. MDF mitres,, when properly glued, hold together very well, especially in paint grade situations, when you can cheat with some caulking. But, yes, unglued mitre will very likely open up, even with MDF. By contrast no glue in the world will keep a wood joint together if it wants to move.
I thought the purpose of coping was to make the joint look cleaner? Humidity here is relatively constant: Around 50-60% in summer and mid-higher 60% in winter so no huge swings as in colder parts. If there's no advantage, I'll just mitre.

Thanks.
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[OP]
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smitty9999 wrote: Maybe it's just my lack of skill but I was never able to cope MDF cleanly. It always chipped.
You and me both. I think MDF is brittle too.
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Sr. Member
Jun 26, 2019
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thriftshopper wrote: You and me both. I think MDF is brittle too.
Sadly, I have next to no experience in MDF because I don't use it, or when I do its for a small wall molding or something thats just a mitre cut.

That said, I think a lot of people who use MDF more often have gone to the angle grinder + flap disc for this reason.

And just to repeat what's been said before, do this outside with a respirator on.

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