Best methods for finishing MDF

[joekraska]

Re: Best methods for finishing MDF

Then applied 3 coats of Urethane Enamel paint with a foam roller....letting 24 hours dry time go by between coats....then sanded smooth and did 5 coats of clear Minwax Urethane gloss....wet sanded to 1500 grit using minimal water with spray bottle.....followed by buffing out with Mequire's rubbing/polishing compound.

8 coats. Now that's dedication. Pictures?

Joe.

[johnnail]

Re: Best methods for finishing MDF

John,
You bring up a good point. For painting the seam will show through, eventually, but if you were to do a miter, then veneer it, fill and then paint. This may work pretty well. The seam will only show a slight crack, but the expansion and contraction will be even between the two pieces and the veneer should bridge the joint.
Dave

Actually, I have a pair of 3 ways, WATT/Puppy style that all seams are mitered....then did a simple edge sanding to take the sharp edges off....possibly a 1/8" roundover done with simple hand sanding. Then applied 3 coats of Urethane Enamel paint with a foam roller....letting 24 hours dry time go by between coats....then sanded smooth and did 5 coats of clear Minwax Urethane gloss....wet sanded to 1500 grit using minimal water with spray bottle.....followed by buffing out with Mequire's rubbing/polishing compound. Did that over two years ago....still no hint of a seam showing anywhere. I did not bother coating the inside of the cabinet walls with anything to seal it. So....I guess I was just LUCKY on that?
For those interested.....the Urethane paint I used was simple Home Depot Glidden Oil-based Urethane Enamel Porch and Patio paint. Tough stuff.

John

[williamrschneider]

Re: Best methods for finishing MDF

Bob Bartko, whose experience I respect, says this in post #55 of this thread...

"It will show.
Right down the middle of the roundover."
"Hopefully one of Dave's experiments will yield some good results.
My vote goes for the trench and fill tactics"

Bob doesn't shoot from the hip, so I suspect there's truth in his claim.

In both of my cases where the seams telegraph through, I estimate it was a week or more before I began to veneer them. In the first case, I had painted the cabinet, but changed my mind and sanded off all the paint to bare MDF. That case was probably several weeks before veneer.

[dlneubec]

Re: Best methods for finishing MDF

I just posted this question to Bill, but I wonder if you chamfer (or use a large roundover) across the a joint so that the joint falls near the middle of the chamfer, if you can effectively do the same thing, since it essentially changes the seam from a "side" to "end" grain to an "end" to "end grain orientation and their rates of expansion and contraction should then match. Obviously you would have the extra effort to cleat the inside corner for strength, etc. but that is certainly pretty easy to do.

[dlneubec]

Re: Best methods for finishing MDF

Yes, I use the 10mil backed veneer. The total thickness of my veneer is 0.025" plus whatever glue line there is to attach it to the cabinet.

Having a thicker veneer should help distribute the emerging mismatches to reduce the effect. On both of my problem cabinets, the effect is slight, and visible only in glancing light. Having even 10 mil veneer must help because it's better than my painted cabinet on another project.

Hmmm, I just checked my ZMV5 cabinets, and the telegraphing seam is NOT visible right now. I saw it in summer though. I'm sure it will be back when humidity rises.

Another observation... I have some stacked "laminated" MDF joints (grain lies in the same direction), and there's absolutely no problem with the paint after 3 or 4 years in any season.

What prep did you do on the mdf prior to veneering and what veneering glue did you use? Do you recall how long the boxes were allowed to stabilize before treatment and/or veneer. Perhaps one method has more telegraphing than another. I've read that contact cement allows some movement, so perhaps it allows the veneer move enough so as to not show the seam as well. Just thinking out loud here.

As I've said I can't see any telegraphing in three of my projects I've checked that are about 1 to 3 years old. In all cases, I've had the speakers built and used them for at least a month inside while voicing, so perhaps they had time to stabilize and that has helped. On two of the three, I used contact cement and one was the Heatlock glue. The ones that had contact cement were sealed with sanding sealer, IIRC.

Your comment about stacked laminates brings to mind another question I have. what happens with a roundover or chamfer that crosses a seam with any of these techniques. I'm not sure any of the methods, other than applying veneer over it, address this situation. Perhaps it is less of a problem, especially with a chamfer(maybe even a roundover), because it begins to turn the joint into two end grains coming together, like a laminate, and less a hard surface to end grain joint. Do you have any experience with that? On my current project I have larger chamfers that put the seam near the middle of the chamfer and I'm wondering how it is going to react.

[davepellegrene]

Re: Best methods for finishing MDF

My favorite method of covering endgrains of MDF is to use Mitered joints. Works every time. Sorry, but it does. No extensive treatment necessary.
If your table saw and miter gauge are trued up nicely, then it is a breeze.

John

John,
You bring up a good point. For painting the seam will show through, eventually, but if you were to do a miter, then veneer it, fill and then paint. This may work pretty well. The seam will only show a slight crack, but the expansion and contraction will be even between the two pieces and the veneer should bridge the joint.
Dave

[johnnail]

Re: Best methods for finishing MDF

My favorite method of covering endgrains of MDF is to use Mitered joints. Works every time. Sorry, but it does. No extensive treatment necessary.
If your table saw and miter gauge are trued up nicely, then it is a breeze.

John

[williamrschneider]

Re: Best methods for finishing MDF

Bill,

Based on your experience, I wonder if 20mil paper backed veneer would work. Was your experience with the typical 10mil?

Yes, I use the 10mil backed veneer. The total thickness of my veneer is 0.025" plus whatever glue line there is to attach it to the cabinet.

Having a thicker veneer should help distribute the emerging mismatches to reduce the effect. On both of my problem cabinets, the effect is slight, and visible only in glancing light. Having even 10 mil veneer must help because it's better than my painted cabinet on another project.

Hmmm, I just checked my ZMV5 cabinets, and the telegraphing seam is NOT visible right now. I saw it in summer though. I'm sure it will be back when humidity rises.

Another observation... I have some stacked "laminated" MDF joints (grain lies in the same direction), and there's absolutely no problem with the paint after 3 or 4 years in any season.

[bmaupin]

Re: Best methods for finishing MDF

I've seen a 0.010" change in the thickness of the MDF, and less of a change in the other dimensions. Drier weather causes shrinkage. I'm still taking measurements periodically, and we'll see what happens when damp spring conditions arrive.

I experienced the same thing recently while building my Overnight Sensations variant - the boards that had sat in the garage were a surprisingly bit thicker than the project boards that had sat inside.

[dlneubec]

Re: Best methods for finishing MDF

I'm very pessimistic about that working. I've seen joints telegraph through maple veneer on two builds. Both have a glossy surface - one with polyurethane and the other finished with Danish oil - and the gloss amplifies the problem.

The joints on both builds were invisible initially, but changing ambient conditions caused the joints to appear as slightly uneven areas on the surface. In recent projects, I'm now using two layers of paper-backed veneer combined with lower gloss finishes to minimize telegraphing of the joint.

As a side note, here's something interesting and perhaps relevant...

While Dave has been doing this experiment, I've been paralleling it by measuring a couple pieces of bare 4 x 6 x 3/4" MDF to determine changes in the size of the material. Since early December, I've seen a 0.010" change in the thickness of the MDF, and less of a change in the other dimensions. Drier weather causes shrinkage. I'm still taking measurements periodically, and we'll see what happens when damp spring conditions arrive.



The finding so far verify what Dave has suspected - thickness changes much more than the other dimensions of MDF. I used two MDF specimens - one tan MDF, the other yellowish - to see if there is much difference between the two "species". There isn't.

Bill,

Based on your experience, I wonder if 20mil paper backed veneer would work. Was your experience with the typical 10mil?

Still, I can't say I've noticed any seams telegraphing through the veneer on my unpainted projects, though they all have high a gloss poly finish. I don't even see any on my SoundRounds veneer, which have a very thin, unbacked veneer over mdf and they are nearing 2 years old now. Maybe it is just more evident on painted finishes. I'm starting to think I should just stay with a wood veneer finish rather than paint. Paint sounds like too much work, with a pretty unpredictable result!:eek:

[williamrschneider]

Re: Best methods for finishing MDF

Frankly, I'm holding out hope that using an inexpensive sheet of tight grained paper back veneer will be the easiest method overall and will adequately hide seams, but I'll have to test that as well.

I'm very pessimistic about that working. I've seen joints telegraph through maple veneer on two builds. Both have a glossy surface - one with polyurethane and the other finished with Danish oil - and the gloss amplifies the problem.

The joints on both builds were invisible initially, but changing ambient conditions caused the joints to appear as slightly uneven areas on the surface. In recent projects, I'm now using two layers of paper-backed veneer combined with lower gloss finishes to minimize telegraphing of the joint.

As a side note, here's something interesting and perhaps relevant...

While Dave has been doing this experiment, I've been paralleling it by measuring a couple pieces of bare 4 x 6 x 3/4" MDF to determine changes in the size of the material. Since early December, I've seen a 0.010" change in the thickness of the MDF, and less of a change in the other dimensions. Drier weather causes shrinkage. I'm still taking measurements periodically, and we'll see what happens when damp spring conditions arrive.



The finding so far verify what Dave has suspected - thickness changes much more than the other dimensions of MDF. I used two MDF specimens - one tan MDF, the other yellowish - to see if there is much difference between the two "species". There isn't.

[dlneubec]

Re: Best methods for finishing MDF

The 1/8 stuff has the benefit of being homogenous, with both dense outer layers in tact. An 1/8 thickness of that is exponentially stronger than the 1/8 of rabbett.

When the skin is applied to an established base (an enclosure in this case) the skin would have plenty of support. It will conform somewhat to the base, but will bridge minor gaps without breaking, unlike a thin sliver of rabbetted mdf.

Way back when I learned that mdf was not well suited for most traditional joinery. Once you cut through the face it loses a lot of its strength.
Grooves, dados, rabbetts, some splines, t&g, etc are all pretty poor choices for mdf. They tend to produce a very weak joint rather than a stronger one.
You can test this very easily. Make a rabbett like in your example and press gently on the end. Watch the piece flake off.
Now rest an 1/8 mdf or hdf piece of same size on a bench with the same amount hanging over the edge. I didn't break with even with a good amount of pressure did it?

For cutting that joint you have 4 reference or critical dimensions instead of two. If one length is off trimming will affect the overall size of the panel which can get you in trouble real quick.
And the panels must be sized very accurately before you cut the joints, unlike a skin with those b(censored) joints or other b(censored) joinery methods.

Doable for many, but for those without good saws and quite some experience it would be a real challenge.

Anyway, either method is more work.
I'm liking the trenches!
We'll see come September. ;)

I think we will have to agree to disagree on this one, though I'll give your test a try. I think you might be overstating the weakness of the 1/8" lip of mdf. I have to admit that I don't recall using thicknesses smaller than 1/4" in the past, so perhaps a 1/8"x5/8" lip would be too weak. My guess is that if you have at least one hardened side intact, it will be sufficiently rigid to hold up unless you drop it on the floor, which would damage a 3/4" piece or 1/8" skin of any size anyway.

In general, getting the dimensions correct is just not that hard, IMO. You simply have to get the depth of the dado blade or router bit setup correct once for each side of the joint and can usually run multiple pieces through with just a couple such setups, using scrap test pieces to ensure the setup is correct. If you run all your joints of the same dimension with a single setup, then there is no reason for them not to fit perfectly, assuming you did an adequate test run on a couple scrap pieces.

I've used rabbets with most of the speaker builds and find them easier overall, when you consider accurately clamping and gluing them together after they are cut, and stronger than b##t joints. Either way you have to get at least one of the panels cut straight and to the right size since they can't all overlap. If you can cut a good 90º angle and get one panel cut the correct size, there is no reason you can't do them all accurately, unless you are in a hurry, are careless or simply make a mistake. Adding rabbets is no big deal if you have a decent router and router table or decent table saw and ideally a dado blade. IMO, they make the panels much easier to get aligned, striaght and square when assembled than trying to do so with b##t joints.

I'm sure the skin concept can work well, but it is also requires extra time and steps, just like rabbet joints, requires a larger, probably heavier cabinet for the same internal volume and requires you to pick up and store both 3/4" material and 1/8" material. Ditto the trench concept, which requires extra hand sanding of the trench, sealing, filling, sanding, etc. Pick you poison, I guess. None come without costs in time and/or cash.

Frankly, I'm holding out hope that using an inexpensive sheet of tight grained paper back veneer will be the easiest method overall and will adequately hide seams, but I'll have to test that as well.

[bobbarkto]

Re: Best methods for finishing MDF

The 1/8 stuff has the benefit of being homogenous, with both dense outer layers in tact. An 1/8 thickness of that is exponentially stronger than the 1/8 of rabbett.

When the skin is applied to an established base (an enclosure in this case) the skin would have plenty of support. It will conform somewhat to the base, but will bridge minor gaps without breaking, unlike a thin sliver of rabbetted mdf.

Way back when I learned that mdf was not well suited for most traditional joinery. Once you cut through the face it loses a lot of its strength.
Grooves, dados, rabbetts, some splines, t&g, etc are all pretty poor choices for mdf. They tend to produce a very weak joint rather than a stronger one.
You can test this very easily. Make a rabbett like in your example and press gently on the end. Watch the piece flake off.
Now rest an 1/8 mdf or hdf piece of same size on a bench with the same amount hanging over the edge. I didn't break with even with a good amount of pressure did it?

For cutting that joint you have 4 reference or critical dimensions instead of two. If one length is off trimming will affect the overall size of the panel which can get you in trouble real quick.
And the panels must be sized very accurately before you cut the joints, unlike a skin with those b(censored) joints or other b(censored) joinery methods.

Doable for many, but for those without good saws and quite some experience it would be a real challenge.

Anyway, either method is more work.
I'm liking the trenches!
We'll see come September. ;)

You may be right Bob. I haven't tried it, but I was comparing it to using a 1/8" mdf skin, which I would think would be as subject to breakage and more subjec to deformation, but certainly easier to cut.

I agree that precision would be ideal, but since we are comparing it to hand sanding valleys in he mdf and then filling it with something, sanding that down, etc., I'm not convinced it is the harder or more time consuming road, even if the joints aren't perfect and need a little filling. It would come down to getting the initial setup right for the rabbet depths and once that is done on some scrap pieces to perfection, it should be just running the parts through, either with a dado bit or router on a table. It just doesn't look that hard to do to me. The one lapped piece could be cut a little larger, allowing for flush bit trimming after, if so desired.

[dlneubec]

Re: Best methods for finishing MDF

Dan,

Rabbets that long and thin in mdf will break very easily. They'll also distort very easily during assembly.
I'd expect a lot of spoilage during construction.
A double rabbett is also a very demanding joint to cut. You have to maintain extreme precision in every dimension.

You may be right Bob. I haven't tried it, but I was comparing it to using a 1/8" mdf skin, which I would think would be as subject to breakage and more subjec to deformation, but certainly easier to cut.

I agree that precision would be ideal, but since we are comparing it to hand sanding valleys in he mdf and then filling it with something, sanding that down, etc., I'm not convinced it is the harder or more time consuming road, even if the joints aren't perfect and need a little filling. It would come down to getting the initial setup right for the rabbet depths and once that is done on some scrap pieces to perfection, it should be just running the parts through, either with a dado bit or router on a table. It just doesn't look that hard to do to me. The one lapped piece could be cut a little larger, allowing for flush bit trimming after, if so desired.

[bobbarkto]

Re: Best methods for finishing MDF

Dan,

Rabbets that long and thin in mdf will break very easily. They'll also distort very easily during assembly.
I'd expect a lot of spoilage during construction.
A double rabbett is also a very demanding joint to cut. You have to maintain extreme precision in every dimension.

Dave,

...

Here's a construction thought. Can we not get the same benefits as the 1/8" mdf or hdf surface by using deep rabbet joints, say a 5/8" depth, leaving 1/8" of material left from the corner? Better yet might be doing a "double" rabbet joint (see below), where both pieces are rabbeted and the legs of the rabbet leave only 1/8" of material for the overlap in both directions. That way you have a seam 1/8" from the corner and you have only 1/8" of material to swell or shrink. With less material to enable shrinking and swelling, might that not reduce the effect? Especially since that 1/8" of material could very easily be permeated with glue and finish material, making it harder for the moisture content to change over time.