Best Way to Glue MDF Miter Joints

[BrianBDH]

A co-worker asked me to help repair a door to a gun cabinet he bought second-hand. The door is 18" wide x 48" tall and is made from MDF veneered frame with a glass panel. The corners are mitered and all 4 joints have failed. The previous owner screwed some crappy looking plywood blocks over the joints but that didn't work either.

My plan is to take the door apart and use buiscuits and glue to properly bond the corners. I haven't glued a lot of MDF, especially miter joints, and would like to get advice on the best glue or gluing procedure.

I have seen posts that mention sealing the ends with glue and then gluing the joint. I have also seen posts that mention Titebond molding glue.

These joints need to be pretty strong as this glass panel is heavy. I am even wondering if an L-bracket in addition to the bicuited joint wouldn't be a good idea. They wouldn't be seen with the door closed.

Thanks in advance for the help.

Brian

 

[ehpoole]

By far the best approach to gluing mitered MDF joints is to employ the use of biscuits, dowels, or splines. These will provide a proper glue surface to reinforce the MDF joint.

There's really no trick with respect to treating the ends of a piece of MDF that will result in a strong joint as your are largely gluing loose sawdust to loose sawdust in the weakest possible manner. Some tricks may be stronger than nothing, but no trick treatment will top those suggested above.

Ideally the manufacturer of the piece should have probably employed lap joints (or lap-miter joints) for their rails and stiles to ensure adequate mechanical strength.

 

[KenOfCary]

Is it impractical to replace the MDF with real wood? Might be hard to match the veneer but might be worth the effort.

 

[Mike Davis]

Yep, a new door of real wood properly jointed will last a lifetime vs. a lifetime of repairs to inferior material.

 

[BrianBDH]

I agree that a real wood frame would be a better solution. But we've all heard the expression about putting "lipstick on a pig". He only paid $30 for the whole gun cabinet off Craig's List, so it might be a solution down the road, but not until we try the cheap route first.

I have also come across posts that say CA glue is best for MDF but the 5-10 second setting time gives me some concern. I am leaning toward sealing the ends with thinned Titebond II and using the biggest bicuit I can along with full strength Titebond II.

 

[BKind2Anmls]

A butterfly joint with a template and small router wouldn't take too long to do , but it may be more work than you want to do.

A spline would work if the MDF is thin and you are afraid the expanding biscuit will telegraph to the surface.

For real quick-and-dirty they have those squiggly short pieces of metal (senior moment, I forget what they are called) that some people used to hold picture frames together. These you just tap into the joint.

 

[tryingtokeepmyfingers]

I have also come across posts that say CA glue is best for MDF but the 5-10 second setting time gives me some concern.

two things

1 in terms of molding glue I think it is thicker and more tacky then regular wood glue


2. in terms of CA I think the thick CA Has bit longer working time... I used it on some mdf quarter round and it worked wonderful and turned out very strong... so I would try thick ca

 

[Berta]

The biscuits will telegraph through. No doubt in my mind. I would use a spline and metal 'L' brackets on the inside if you want to reuse the MDF. I think it would be easier to build a new one out of popular and paint it.

 

[golfdad]

what is Ca glue is that like Gorilla glue?

 

[Jeff]

+1 to a real wood door but understand your reasons for not going there yet.

IMO, a quality epoxy may work better than either TB or CA because epoxy has excellent gap filling properties. Mating miter joints may be 44.9 and 45.1 degrees but that's ok because they're complementary and still total 90 degrees. So it's important to letter/number those mating joints before disassembling the door.

Clamping miters is a slippery slope but these universal corner clamps work very nicely with some practice.

http://www.leevalley.com/en/wood/page.aspx?p=49996&cat=1,43838,55979

 

[KenOfCary]

what is Ca glue is that like Gorilla glue?

CyanoAcrylate (forgive my spelling) also sometimes called Super Glue. Gorilla does make CA glue but it is not the expanding foamy stuff that you're likely thinking of.

 

[Rick M]

I wouldn't overthink it, just go with wood glue and biscuits. Like you say... lipsticks and pigs, it's a $30 cabinet.