MDF Seam Test Box with Trenches

[davepellegrene]

Since my test pieces, with trenches, seem to be doing good so far I decided to try a full box test. I made two boxes. One I glued up last weekend and will allow at least 72 hours between every processe. The other box I am only going to allow enough dry time to work the products. Probably the longest period will be the primer. I will allow 24 hours.

I used this bit to cut the trough.

I cut the trough around the baffle 3/32" deep. I rounded over the front baffle with a 1/2" round over. I went small on the round over because all I had was 1/2" MDF laying around and didn't want the round over to get into the trough. I wasn't going to round over the top of the baffle, but did it with out thinking. Had I done the trough a little shallower like I did the trough on the box seams, 1/16" deep, I probably could have done a 3/4" round over.


Here's a pic of the bottom. I didn't round over the front baffle on the bottom.



I decided to use Wolf's hand rubbed wood glue method on all the exposed end grain. My thinking was if moisture gets into the MDF the wood glue may hold the end grain from expanding and showing the seam between the two 1/2" baffle panels. I used just enough to rub it in without it running or building up on the panels. This was to much. One bead is plenty.



After the glue dries completely I will sand with 120 sand paper to make sure I get a good bond to the glue with the body putty.
I kind of like the look of the trough around the baffle. I may use this on a build sometime.
Dave

[lunchmoney]

Re: MDF Seam Test Box with Trenches

Are you eventually going to fill the trenches with something? Bondo?

[davepellegrene]

Re: MDF Seam Test Box with Trenches

Are you eventually going to fill the trenches with something? Bondo?

Yes. I did say that at the bottom of the post. You must have missed it.
Dave

[lunchmoney]

Re: MDF Seam Test Box with Trenches

I was following your first thread and all this is interesting, but it makes me wonder why. Why bother? I don't understand.

Because mdf is far cheaper.

I've created very nice painted finishes on mdf.

Yes, the seams do eventually show ever do slightly. Acceptable in my opinion, and worth it considering the amount of money I saved by not using plywood. The seams are only a problem if you're being hyper-critical and looking for problems. If anything less than 100% perfection = failure, then I guess mdf fails.

I also don't see how painting plywood would be any easier than painting mdf... don't seams also show with plywood?

[lunchmoney]

Re: MDF Seam Test Box with Trenches

Yes. I did say that at the bottom of the post. You must have missed it.
Dave

Yep, sorry... missed it.

I suppose the question now becomes whether you'll see seams where the bondo meets the mdf.

Eagerly awaiting the results!

[davepellegrene]

Re: MDF Seam Test Box with Trenches

I was following your first thread and all this is interesting, but it makes me wonder why. Why bother? I don't understand. MDF is not suitable for long term high quality finishes - IMO. Plywood is far superior for speaker building in just about every way.

I guess I'll have to do my own strength and moisture studies on a small range of wood sheet products. Here's a spoiler based on personal experience - MDF is going to come in last on every front including price/performance.

I would much rather use MDF for a base to paint.
1. Reason not to use plywood is the grain. Unless you want the grain to show through you would have to fill it all in.
2. Plywood splinters.
3. just as much work to fill in the end grains of plywood.
4. MDF is more dense
5. Plywood has voids in it. Unless you buy high quality and that will cost twice as much.
6. I am not so sure the seams would not show through on plywood.
The seams are not that bad when they do show up. I am pursuing this more so for high end finishing. If you spend the time and money to put on a nice finish it is aggrevating for the seams to show. Just the few builds I have done there is an amazing amount of time involved, so why not a few more hours to hopefully get a better end result.
Just my opinion.
dave

[davepellegrene]

Re: MDF Seam Test Box with Trenches

Yep, sorry... missed it.

I suppose the question now becomes whether you'll see seams where the bondo meets the mdf.

Eagerly awaiting the results!

So far on my samples that the bondo is directly on the mdf no seams as of yet. I wanted to get this box built before we have a season change so it will go through it. That I think will be the final test.
Dve

[lunchmoney]

Re: MDF Seam Test Box with Trenches

If you consider that plywood is almost 2x stronger than the same thickness of mdf then mdf is actually more expensive.

That might be a relevant argument if the material actually needed to be that strong. It doesn't. 3/4" MDF is plenty strong enough for most applications, particularly if you add a few simple braces. I don't beat on my speakers... do you?

Perhaps you're arguing that I could use 3/8" plywood instead of 3/4" mdf? Maybe, but 3/8" furniture grade plywood (yes, furniture grade, so that its free of rattling voids), is still way more expensive than 3/4" mdf.

Please post pix of your seam-free painted plywood speakers.

[davepellegrene]

Re: MDF Seam Test Box with Trenches

Birch plywood glued, clamped and nailed then primed and painted to a poplar edging. Seam showing all the way around. This spot is the worse. Have you ever put a high gloss finish on a piece of plywood. It is wavy as can be. No were near as flat as MDF.
You then have the issues of the plywood delaminating from the finish. Sometimes it delaminates on its own. None of this is noticeable when stained and finished because the grain hides it all.
Dave

[bobbarkto]

Re: MDF Seam Test Box with Trenches

...

I've never seen spontaneous delamination in reasonable quality plywood. Sounds like a rare manufacturing defect to me.

It happens. Even with premium, domestically produced panels.
Voids are an issue if resonance is a concern.
Good, genuine, baltic birch or marine ply or die-board is probably equal to mdf in consistent density.

...And I'm not sure what you mean by a wavy finish. There's absolutely no reason it should be wavy. If it is you aren't finished yet. You need more coats, then you need to sand it flat. Then it won't be wavy.

It still takes more work to get a glass smooth finish on practically any wood. Any grain will require filling.
How many coats of primer or surfacer? How much sanding? More than mdf that is for certain.
Finishing materials cost money. Add that to the cost of good plywood and the labor involved in getting mdf ready is a wash. Plus you get a better enclosure all things being equal.

And plywood moves nearly just as much as mdf does in both width and thickness. It is species and thickness and # of plies dependant, but the differences in coefficient of expansion are, on average, small. Not really an issue in this context. Both products are probelmatic in this regard.

[ckmoore]

Re: MDF Seam Test Box with Trenches

Dang Dave, when you were talking about the trench method, I was thinking in my minds eye of a MUCH shallower trench. But those are DEEP! Don't get me wrong, I was thinking the same thing as you; that could be a nice design element... I like it.

[Wolf]

Re: MDF Seam Test Box with Trenches

Dang Dave, when you were talking about the trench method, I was thinking in my minds eye of a MUCH shallower trench. But those are DEEP! Don't get me wrong, I was thinking the same thing as you; that could be a nice design element... I like it.

I like it too, and he should just go ahead and use those boxes for a project...
Later,
Wolf

[ckmoore]

Re: MDF Seam Test Box with Trenches

I was just thinking, don't you run the risk of cracking with bondo applied that thick?

[donparsons]

Re: MDF Seam Test Box with Trenches

2nd paragraph - I could not disagree more. Have you ever soaked a piece of mdf in water? A 3/4 inch thick piece will swell up to well over 2 inches thick. That absolutely does not happen with plywood. It also swells in width - not nearly as dramatically but a lot more than plywood.

Are you sure it wasn't one of those compressed sponges you are talking about? Well over 2"? Come on.

[davepellegrene]

Re: MDF Seam Test Box with Trenches

Dang Dave, when you were talking about the trench method, I was thinking in my minds eye of a MUCH shallower trench. But those are DEEP! Don't get me wrong, I was thinking the same thing as you; that could be a nice design element... I like it.

Yeah, I got the trenches too deep. I didn't realize they were that deep until I was done. Probably to deep for finishing putty, but should be ok for body putty. Its just going to take more work to get it flat. Another good reason for "practice" boxes.