Best methods for finishing MDF

[dlneubec]

Re: Best methods for finishing MDF

What has been explained to me is that it is do to moisture content in the MDF. Low moisture it shrinks. High moisture it expands. Being that it is compressed most of the expansion/ contraction is in the thickness. In this drawing, not very high tech sorry, you can see two lines drawn for the side panel one is if the moisture content is low, contraction. The other if moisture content is high, expansion. At the point the two pieces are glued together the glue holds the joint from moving. Its just past the glue joint that the MDF expands/ contracts, rather sharply that you get the line.


I think it has a lot to do with how much moisture is in the MDF when you sand and finish compared to how much when it is sitting in the room. I built two subs at different times with the same process, automotive finish. One has the side panel slightly back to show the seam and the other sticks out slightly to show the seam. So, I think there was more moisture in teh one recess back during the build and less moisture in the one sticking out then what is in my house now.

This is why I am trying the trench idea. If I can come up with a filler that will allow the MDF to expand without cracking it may work.
Dave

You are probably right Dave, but I wonder if we can do something different in our construction techniques to mitigate the potential problems(other than mitre joints), rather than go to elaborate efforts afterwards.

For example, I wonder if a very deep rabbet joint, with maybe just 1/16" to 1/8" of material remaining on the outside piece would show less than a standard **** joint, since it the joint is very near the corner and the glue would likely permeate that narrow section quite well. Obviously a narrow lip like this would be very easy to break off, so that would certainly be a potential drawback.

Also, rather than doing roundovers, might one do chamfers and place the chamfer edge right on the seam. Might that mask the effect of expansion?

Is the amount of expansion changed by the surface hardness of the mdf? Could it be sanded to remove the harder layer at the surface and in that way reduce the potential expansion?

I think this kind of construction technique options might be a good addition to your testing of treatments, if you are interested in exploring them. It just seems to make sense to try and address the problem at the source rather than treat it afterward. A combination of a change in construction techniques and simple seam treatment may offer the easiest solution.

BTW, how did you do the trenches?

[davepellegrene]

Re: Best methods for finishing MDF

I assume much of the problem has to do with the difference between the softer, more absorbant edge cut mdf and the hardened surface of the panel adjacent. I wonder if one could sand down the hardened suface enough so that it would act more like the edge cut? Might that make their reaction to surface treatments, humidity, etc. the same?

What has been explained to me is that it is do to moisture content in the MDF. Low moisture it shrinks. High moisture it expands. Being that it is compressed most of the expansion/ contraction is in the thickness. In this drawing, not very high tech sorry, you can see two lines drawn for the side panel one is if the moisture content is low, contraction. The other if moisture content is high, expansion. At the point the two pieces are glued together the glue holds the joint from moving. Its just past the glue joint that the MDF expands/ contracts, rather sharply that you get the line.


I think it has a lot to do with how much moisture is in the MDF when you sand and finish compared to how much when it is sitting in the room. I built two subs at different times with the same process, automotive finish. One has the side panel slightly back to show the seam and the other sticks out slightly to show the seam. So, I think there was more moisture in teh one recess back during the build and less moisture in the one sticking out then what is in my house now.

This is why I am trying the trench idea. If I can come up with a filler that will allow the MDF to expand without cracking it may work.
Dave

[dlneubec]

Re: Best methods for finishing MDF

I assume much of the problem has to do with the difference between the softer, more absorbant edge cut mdf and the hardened surface of the panel adjacent. I wonder if one could sand down the hardened suface enough so that it would act more like the edge cut? Might that make their reaction to surface treatments, humidity, etc. the same?

Alternatively, what is the treatment process for mdf panel sufaces? Can we simulate this treatment on the cut edges in order to make them act more like the surface?

[lunchmoney]

Re: Best methods for finishing MDF

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. ;)

The trench and fill is interesting...

But I would think that you would then simply have two emerging seams, one on either side of the trench.

Two different materials, thus two different behaviors in terms of expansion/contraction... don't have highs hopes for this one... hope I'm wrong...:o

[bobbarkto]

Re: Best methods for finishing MDF

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. ;)

I was interested in this method as well. Anyone out there have experience with this?

[rogoll]

Re: Best methods for finishing MDF

Do you think you could do a roundover on a mitered seam and it will still not show? For a lot of the small two ways, you don't nees anything much thicker than a 3/4" baffle. I haven't done miter cuts yet on any of my cabinets, but may next time I plan to paint. My next two are going to be veneer so I doubt it will matter.

I was interested in this method as well. Anyone out there have experience with this?

[donparsons]

Re: Best methods for finishing MDF

Do you think you could do a roundover on a mitered seam and it will still not show? For a lot of the small two ways, you don't nees anything much thicker than a 3/4" baffle. I haven't done miter cuts yet on any of my cabinets, but may next time I plan to paint. My next two are going to be veneer so I doubt it will matter.

On my Nightmare speakers all the angles are mitered together except for the front baffle and rear panel. None of the mitered seams are showing so far. The problem with this method is that you would have to 45 degree all four sides. With the front baffle usually thicker makes it more difficult. It also makes building boxes a lot tougher to do if you don't have the tools, knowledge or good wood working skills.





Dave

[davepellegrene]

Re: Best methods for finishing MDF

Not sure if these would be as good structurally on a big cabinet but perhaps on a small one?

http://tapeease.com/cabinet1.htm

Those would probably work good for a large radius but you would still have the seam that would swell and show up.
Dave

[davepellegrene]

Re: Best methods for finishing MDF

Dave
What epoxy method are you using? I wrote to tech support at West Systems epoxy to ask them about edge sealing on MDF, they responded that their 105 with 205 hardener would likely do a good job, but I haven't tried it.

Steve

The epoxy I am using came from HD. I am trying to use products easily obtainable.


I have used similar products to the West Systems.
This summer I put a new surface on my waterski jump. I started with a 10 year 7 ply treated plywood.
I used CPES to seal and harden the plywood.
http://star-distributing.com/smith/products.html
I then coated it with two coats of High build epoxy paint.
http://star-distributing.com/smith/hbepwhy.html
Then I put two coats of red boat enamel paint on that. This paint is a two part mixture.
I thought about using this method on speakers, but it is way to expensive to do. That and I am sensitized to epoxy, so if I don't were full protection I get what is like poison ivy mixed with sun burn with lots of swelling. The only way to get ride of it taking steroid dose packets. Not fun, so I try to stay away from it as much as possible. Anyone can become sensitized to epoxy at any time, so becarefull. One you are sensitized its for life.
Dave

[davepellegrene]

Re: Best methods for finishing MDF

one thing that helps, but doesn't completely solve the problem, is to get rid of indented seams altogether. when i build MDF cabinets, i usually try to cut 3-4 extra pieces the width of the top/bottom panels. when you go to cut the side panels, make them 1/4" oversize. when you go to glue, use the extra pieces to square up the top, and let the sides overhang about 1/8". then use a flush trim bit to take them off. it makes for some nice sharp edges. i'll also second wolf's hand-rubbed glue method. and for baffles, do the same -- overcut about 1/4"-1/2", and flush trim the entire thing. i think i used to spend about 20-30% of construction time measuring accurately to make perfect baffles and corners, and now a $17 bit shaves most of that off. rustoleum appliance epoxy paint is also good stuff. goes on nice and thick. it only comes in a couple colors, but you can always paint over it again.

That is a good method to get all the corners nice and straight. A lot faster too.

[davepellegrene]

Re: Best methods for finishing MDF

Excellent, Dave!

Thanks for all your hard work. It'll a tremendous contribution to this community.

Can't wait for the results...

The bad news is that you really won't know for sure what the long term results are for a good 6 months at least

Hopefully when I am all done we will have a base to decide how difficult each method is compared to the final results. If you have a satin finish and don't mind a little seam showing you can choose a simpler less expensive method. Its when you spend a lot of time on a piano finish or something similar and a seam pops through later on. Maybe one of the methods may work. Keep your fingers crossed.
Dave

[davepellegrene]

Re: Best methods for finishing MDF

I understand. I am wondering if the miter, even though it was rounded over would reduce the propensity of the seam to appear.

On my Nightmare speakers all the angles are mitered together except for the front baffle and rear panel. None of the mitered seams are showing so far. The problem with this method is that you would have to 45 degree all four sides. With the front baffle usually thicker makes it more difficult. It also makes building boxes a lot tougher to do if you don't have the tools, knowledge or good wood working skills.





Dave

[donparsons]

Re: Best methods for finishing MDF

I wouldn't want a seam to emerge right smack in the middle of the roundover.

I understand. I am wondering if the miter, even though it was rounded over would reduce the propensity of the seam to appear.

[lunchmoney]

Re: Best methods for finishing MDF

We know that miter cuts will eliminate this problem. I mentioned that to Lunch in one of these converstations and he said he likes roundovers. I have been wondering what would happen if you did a roundover on a miter cut. You would only have one seam to measure and it is the seams that show not the end grain. What say you?

I wouldn't want a seam to emerge right smack in the middle of the roundover.

[donparsons]

Re: Best methods for finishing MDF

We know that miter cuts will eliminate this problem. I mentioned that to Lunch in one of these converstations and he said he likes roundovers. I have been wondering what would happen if you did a roundover on a miter cut. You would only have one seam to measure and it is the seams that show not the end grain. What say you?