Best methods for finishing MDF

[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

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

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. ;)

[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

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:

[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.

[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.

[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

[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

[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.

[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.

[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.

[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

[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.

[davepellegrene]

Re: Best methods for finishing MDF

I looked on my Mini Statements this morning. They have a 3/4" round over with backed veneer and satin finish varnish. I don't see a seam on most of the round over, but I do see a slight dip in a few spots. I did not seal them. I put two coats of Titebond II on them. The bases are made of four layers of 3/4" MDF with several coats of Bin 123 then sprayed with acrylic enamel with hardener. If I look close I can see the seams between the pieces. I had already thought about rounding over past the seam.
On my Nightmare speakers I used Miniwax Hardener, Several coats of Bin 123, Two coats of Waterborne auto primer, base coat and then clear coat. I can spot the seam on the 1 1/4" baffle. These seams showed up within a few hours of the final clear coat. Even the mitered seams show, but you really have to look close to spot them. The inside curve on them were filled with auto body putty and they are not showing a seam.
In all fairness I did not allow much time between all the coats, so the moisture was probably the culprit.
The samples I did that have a very shallow, about 1/32" deep, trench are not showing the seam much at all. You could do that then veneer over it. Here is a pic of the four pieces. The one that looks the worse is the one I used fiberglass resin thinned 40/60. It soaked up 5 ounces. It swelled enough to almost eliminate the trench. Had I let it dry for a week or so it may have been ok. This is after 28 days, on the shower shelf for two weeks and back out for about a week.

Here is the body putty in the trench. No sealer, primed over the putty then two coats of paint. The ends and back or bare MDF. This was also on the shower ledge. No seam so far.

Here is the finishing putty in the trench. Same set up as the body putty. No seam so far.

All the samples I have been working on are posted on my link below. Scroll to the bottom of the page. It shows each sample at each stage.
Dave