Best methods for finishing MDF

[davepellegrene]

Re: Best methods for finishing MDF

Dave,

Thanks again for the work you are doing. When you get a chance please consider constructing a step by step instruction on what you think is the best proceedure to minimize the effect of seams, based on what you have learned. That would include a methodology when you don't do the trench method. You have much of it in the post above, but it is hard to follow. For example, how long do you suggest waiting after glue up before sanding. Would you sand smooth, apply sealer, let set 72 hours and sand again. I'm sure you see what I'm getting at.

Yeah, I didn't expect it to turn into a 7 page thread. I sure wouldn't want to read through the whole thing. I will write up each method i have tested then someone can choose what they feel will work for them or at least give them something to build from.

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.

I do think the rabbet joint would work better then gluing the 1/8" panel on the side, especially the larger the panel, as long as you don't damage the edge in the process. Coupled with dry time it would probably hid very well. Its like coke with lime said 30 days to completely dry.
I came up with 72 hours between coats or after gluing up from installing hardwood flooring. They tell you to let it sit in the area to be installed for at least 72 hours before install to acclimate. This seems to be close to the time it takes the MDF to do 90 % of its shrinking after moisture is introduced.
I think I am also going to do a sample with veneer to see how that hides the seam. I think the veneer would work well on the end seams of several layers glued together to make a thicker piece. Not sure if it will bridge the b**t seam. Maybe with enough dry time before installing.
Dave

[CokewithLime]

Re: Best methods for finishing MDF

I guess I should have named this thread" Best Method for Hiding the Seam in MDF". That is what I am trying to accomplish.......
All in all the biggest thing I find in any of the methods is to allow plenty of time for the moisture content to dissipate from the MDF during the finish process. I would say 72 hours or more at 70 degrees. I would also add it would be a good idea to sand after 24 hours. This will allow the finish to breath better to allow the moisture out.

Dave - my daughter works in a coatings lab - she has often admonished me for being too impatient when finishing projects.

The following info is what she refers to and is provided by manufactures that produce glue, paint or any "wet" based coating - ie water, oil or solvent based.
- most will surface "dry" within a 8 to 12 hour period, however to "cure" to the manufactures target hardness and final long term moisture content typically takes up to 30days at a nominal 70 degrees and 40-50% relative humidity.

It seems like your tests (which are every much appreciated, btw !) are pointing to general agreement with manufactures recommendations.

I guess by nature we are all just too impatient in our enthusiasm with a new project.

[dlneubec]

Re: Best methods for finishing MDF

Dave,

Thanks again for the work you are doing. When you get a chance please consider constructing a step by step instruction on what you think is the best proceedure to minimize the effect of seams, based on what you have learned. That would include a methodology when you don't do the trench method. You have much of it in the post above, but it is hard to follow. For example, how long do you suggest waiting after glue up before sanding. Would you sand smooth, apply sealer, let set 72 hours and sand again. I'm sure you see what I'm getting at.

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.

[davepellegrene]

Re: Best methods for finishing MDF

I guess I should have named this thread" Best Method for Hiding the Seam in MDF". That is what I am trying to accomplish. From what I have learned there are many ways to seal and finish MDF. From all the methods I have tested they all work just none, except for the trench method, will hide the seam. I should say so far. Keep your fingers crossed. There are methods of doing the seam differently like Jim Holtz explains, with covering with 1/8". Also cutting the seam on a mitered angle. Both methods put it on the corner so it is less noticeable. In my experience it will still show just much harder to pic out. Both methods are probably less work then the trench method. Covering the seam with veneer may work as well, just seams like quite a bit of work and then you have the wood grain to fill, if you don't want it to show.
All in all the biggest thing I find in any of the methods is to allow plenty of time for the moisture content to dissipate from the MDF during the finish process. I would say 72 hours or more at 70 degrees. I would also add it would be a good idea to sand after 24 hours. This will allow the finish to breath better to allow the moisture out. Another thing I will mention is the grit you chose to sand with. If you try to sand the finish before it is completely dry you are scratching the surface and this could have to do with why a lot of people thing it is necessary to sand with such fin paper. Allowing a lot of dry time between coats will make it sand easier, powder up, and will also help eliminate the finish from shrinking over the next 30 days from all the trapped moisture. I think by allowing more time for the moisture to dissipate will alleviate imperfections from eventually telegraphing through the finish. I learned in doing automotive work if you rushed the repair process a month or so later you would see waves in the finish that were not there to start with.
I will start another thread on my results of the samples I have been working on and rename it so its about the seam and not methods of applying the finish. I will also put a link to it here
Thanks Dave

[ClosetSciFiGeek]

My method for finishing MDF

After multiple iterations of sand and try a new method this has worked the best for me. Best of luck this can be a maddening event to find the right process.

1) Hand Sand any machined edge(like 3/4" roundover or end of **** jouint to 400grit
2) Skim coat with Auto Body Finishing Compound and allow to dry fully
3) Sand to get rid of all small pock marks down to 400grit
4) Wipe down with tack cloth and then acetone until the paper towels aren't pink anymore(usually the finishing compound is pink)
5) Immediately shoot with Bulls Eye 1-2-3 Primer Sealer Stain-Killer
6) Allow to dry overnight
7) Sand to 400 grit(the primer is very thick, but go easy)
8) Wipe down with tack cloth then acetone until no more debris comes onto the paper towel(I use the blue ones from autozone so I can tell when I am done because the primer is white)
9) Immediately shoot Appliance Hard Enamel Paint(this stuff is semi-self leveling and harder for us DIY guys to screw up, plus it is really hard and durable when dry)

[Jim Holtz]

Re: Best methods for finishing MDF

I haven't seen 1/8" mdf around here, but tempered hardboard is common.

Hi Pecker,

My son used to work at a store specializing in hardwoods that also carried all thicknesses of MDF for their commercial customers. 1/8" is only about $8 for a 4'x8' sheet and is much easier to hide the seams with than the endless sanding and hoping the seams won't show through process.

good stuff!

Jim

[pecker]

Re: Best methods for finishing MDF

I've tried a number of different methods of sealing the seams and the only one I've found that is permanent is 1/8" MDF. I cap the ends/surfaces with all of the seams using 1/8" MDF and then you just have a 1/8" seam that is on an edge to worry about.

BTW, bondo is the best solution other than 1/8" MDF I've found for seams.

Jim

I haven't seen 1/8" mdf around here, but tempered hardboard is common.
A couple passes through a thickness planer can get it down to 1/16", and the tempered surface is practically ready for painting...doesn't scratch as easily as mdf.

Ease over the edge with a sanding block and you'll never know it's there.

[bobbarkto]

Re: Best methods for finishing MDF

Finishes will do that, too.
Especially those with alcohol in them, it absorbs atmospheric moisture and introduces it to the substrate.
These effects a rapid and short term. Your advise to wait before sanding is spot on.

Wait for the other humidity effects to show.
With the finished samples, your shower test barely had enough time to allow them to react due to all the finish material. The finish was doing its job keeping out moisture.

Keep them in a dry location now and observe over the course of a year.
My hopes are high! Nice job!!!

Dan,
FWIW, With the tests I have been doing I am coming to the conclusion that the moisture that is causing the seams to show is from the finishing process itself. Every one of the seams that I have had show up on the panels shows that the piece b u t t ing into the other is expanding from the finish applied then shrinking so the end grain is sticking out past. Basically the moisture expands the MDF in its thickness then shrinks again when it disipates. Hope that made sense. The process seams to take a few days. Basically the time it takes for the moisture to dissipate from the MDF. My suggestion, if you are going to try the veneer method. I would seal the MDF, with what ever you chose. The glue you are going to use to put on the veneer would be fine. Then let it sit for about 72 hours for the moisture to dissipate. If the seam shows up fill it then reapply the glue and wait to see if that takes care of it. Then apply your veneer.
Dave

[davepellegrene]

Re: Best methods for finishing MDF

Here's my favorite things about this thread: I thought it was just me! I like painting my speakers and the damn seams keep coming out. I particularly just like to use regular semi-gloss wall paint, to match the trim in the room they're going in (usually white) and I roll it on so it's textures like the wall paint. I use a pretty good quality oil based primer underneath it, and oil paint over it, and I put on more coats than I care to, and they still come out. I build very tight cabinets with invisible glue lines all around, and sand them perfectly to 600, and the seams still come out. I've tried various sealing methods and the seams still come out.

The one set I have painted where the seams don't show is the one I used textured paint on.

Great threat and I hope there's an answer. I build them to listen to, not to look at, but I'm a perfectionist, and it when imperfections develop despite my best efforts, it's frustrating.

So far the seams that are trenched out over a 1/16" deep and filled with either auto body putty or polyester finishing putty are holding up good. The finishing putty is much easier to sand and doesn't clog the paper at all. Doesn't really seem to matter what sealer was used. Even the ones with out sealer with just primer and paint are holding up good.
Dave

[mslatter]

Re: Best methods for finishing MDF

Here's my favorite things about this thread: I thought it was just me! I like painting my speakers and the damn seams keep coming out. I particularly just like to use regular semi-gloss wall paint, to match the trim in the room they're going in (usually white) and I roll it on so it's textures like the wall paint. I use a pretty good quality oil based primer underneath it, and oil paint over it, and I put on more coats than I care to, and they still come out. I build very tight cabinets with invisible glue lines all around, and sand them perfectly to 600, and the seams still come out. I've tried various sealing methods and the seams still come out.

The one set I have painted where the seams don't show is the one I used textured paint on.

Great threat and I hope there's an answer. I build them to listen to, not to look at, but I'm a perfectionist, and it when imperfections develop despite my best efforts, it's frustrating.

[davepellegrene]

Re: Best methods for finishing MDF

That's the method I've been thinking of doing on my current project, where paint is used. A backed veneer should hide the joints effectively, I would think.

Dan,
FWIW, With the tests I have been doing I am coming to the conclusion that the moisture that is causing the seams to show is from the finishing process itself. Every one of the seams that I have had show up on the panels shows that the piece b u t t ing into the other is expanding from the finish applied then shrinking so the end grain is sticking out past. Basically the moisture expands the MDF in its thickness then shrinks again when it disipates. Hope that made sense. The process seams to take a few days. Basically the time it takes for the moisture to dissipate from the MDF. My suggestion, if you are going to try the veneer method. I would seal the MDF, with what ever you chose. The glue you are going to use to put on the veneer would be fine. Then let it sit for about 72 hours for the moisture to dissipate. If the seam shows up fill it then reapply the glue and wait to see if that takes care of it. Then apply your veneer.
Dave

[evilskillit]

Re: Best methods for finishing MDF

Heh, that look on his face says, "My stereo is better than any of my friends stereos, and I know it." He's probably right. I wish I had speakers that nice when I was his age. Hopefully he'll appreciate them, and maybe want to build his own once he gets a job and can afford parts. Ahh, the American dream, father and son, building audiophile speakers together, side by side.

[joekraska]

Re: Best methods for finishing MDF

Would that be the post from mgrabow "Check Out This DIY Build".

Yes.

Joe.

[donparsons]

Re: Best methods for finishing MDF

That's the method I've been thinking of doing on my current project, where paint is used. A backed veneer should hide the joints effectively, I would think.

On my son's tritrix, I used 3/4" red oak ply, and inset the bottom of the front panel and covered with a 1/4" solid red oak panel. So there are no plywood seams visible.

[dlneubec]

Re: Best methods for finishing MDF

That's the method I've been thinking of doing on my current project, where paint is used. A backed veneer should hide the joints effectively, I would think.