Invisible MDF butt joint

[eganders]

I built a speaker enclosure out of MDF attaching the sides with PVA glue and brads using butt joints and spraying the surface with automotive primer-surfacer to get a smooth surface. The result was very good, but there was a SLIGHT hint of the butt joint seam at the overlap where one piece of MDF butts with the surface of the other piece. I think there is a slight difference between the porosity of the end vs. the surface or something between the two pieces. I let the two pieces dry for about a week before sanding the edge flat.

I have been experimenting with different methods to hide the seam for future projects. I have tried:

A washcoat of PVA glue covering the edge and the overlap area (first effort not a spectacular success) Using drywall joint compound to fill the inperfections (was not impressed with the result) A coating of superglue spread over the area (this seems to hold promise)

Has any one tried an approach to stabilize the edge fibers of the cut edge? It would appear that might hold the most promise from the superglue result so far.

Any other thoughts?

[eganders]

I built a speaker enclosure out of MDF attaching the sides with PVA glue and brads using butt joints and spraying the surface with automotive primer-surfacer to get a smooth surface. The result was very good, but there was a SLIGHT hint of the butt joint seam at the overlap where one piece of MDF butts with the surface of the other piece. I think there is a slight difference between the porosity of the end vs. the surface or something between the two pieces. I let the two pieces dry for about a week before sanding the edge flat.

I have been experimenting with different methods to hide the seam for future projects. I have tried:

A washcoat of PVA glue covering the edge and the overlap area (first effort not a spectacular success) Using drywall joint compound to fill the inperfections (was not impressed with the result) A coating of superglue spread over the area (this seems to hold promise)

Has any one tried an approach to stabilize the edge fibers of the cut edge? It would appear that might hold the most promise from the superglue result so far.

Any other thoughts?

[Morris Dovey]

| Has any one tried an approach to stabilize the edge fibers of the | cut edge? It would appear that might hold the most promise from the | superglue result so far.

I've had good results routing both edges straight, flat and square and joining with epoxy only. I cleaned up very carefully with a sanding block - and there was no trace of joints after painting.

Might work for you, too. FWIW, I don't like butt joints in MDF and normally do a M&T-type joint.

-- Morris Dovey DeSoto Solar DeSoto, Iowa USA

[Robatoy]

Automotive glazing putty (sometimes called 'green-stuff' even though it's often red.) works very well. It is lacquer based and sands very well.

[Robatoy]

A rabbet with a mini-mitre works well. (DO get that image of a bunny with a pope's hat out of your mind NOW) Porous-to-porous MDF joints can be very strong if one uses an adhesive with sufficient capillary action to take advantage of MDF's weakness. I have built bass boxes with 4x18" woofers which stood the test of time and drunk roadies for many years. A butt joint to the shiny side of MDF is only as strong as the first layer of surface-tensioned dust....not good.

[Lew Hodgett]

Also known as "spot putty", comes in a tube like toothpaste and is usually red..

Lew

[GROVER]

I have seen professional painters use a product known as painter's putty on the edge surface of MDF. Probably available where the pro painters shop. They apply it with the thumb. Joe G

[Father Haskell]

Shellac or splined miters. Not sure if biscuits won't raise bumps when they swell.

Plastic laminate will hide pretty much anything.

[John B]

G'day, I have used MDF quite a bit for varied projects and painted hundreds of them. Have never had to resort to any exotic fixes to hide butt joints. Just apply a couple of coats of undercoat/sealer, sanding between coats, followed by a few top coats. I use water based acrylic all most exclusively, however the couple of times I have used an oil or thinners based product the results haven't varied much. The thinners based products may need an extra coat or two.

Hope this helps

regards John

[eganders]

I have been over to Parts Express speaker building forum. They have some people that are really into this subject. From their discussion, this problem is more troublesome than I first thought. One of them referred me to this forum:

a bunch of experimenters and quite an interesting discussion!! Read their discussion on using veneer to hide the joint.

By the way, I have tried to find 1/8 inch MDF of the type David Marks uses with little success, but it would be nice to have 1/16 inch MDF to cover the surface that has the butt joint in cases where you were only going to paint the MDF.

Anybody know of an MDF type of material that could be a substitute for veneer where you were only going to paint the surface? A very thin sheet of MDF veneer (or other engineered veneer-like material) would be easier to work with than real wood veneer that has flattening issues. It would have to be very thin (yet thick enough to mask joint movement of the MDF) in order not to have edge joint issues itself.

[Father Haskell]

Cover the joints with stainless angle.

[BobS]

Bondo.......?

[Lew Hodgett]

At one time "Bondo" was a trade name for polyester resin thickened with talc, usually pink, that was used as a auto body filler.

Today, "Bondo" is a generic trade name for a whole group of specialty products including body filler and spot putty which are not the same thing BTW.

Lew