Has anyone mastered hiding joint lines?

[TP143]

Hi everyone,

For the past week I have been testing, testing, and more testing ways to hide joint lines on my mdf build. Eventually I am going for the "piano black" look. So far I have tried wood glue, bondo, spackling and wood filler with no luck. The closest I came was with wood glue, which actually hid the lines but created new lines on each side of the glue. Today I picked up a can of filler primer so I will try that tonight but I was curious if anyone had a better way to hide these things.

Thanks

[ReissM]

Re: Has anyone mastered hiding joint lines?

I saw a design a little while back where the guy beveled the edge exactly far enough so that the bevel ended at the seam/joint.

So for a 45 degree bevel, in 0.75" MDF, the bevel would wind up being 0.75* 1.414 = 1.06 inches.

Clever idea that hides the seams. If my description doesn't make sense, I'll try to draw it. Let me know.

[johnnyrichards]

Re: Has anyone mastered hiding joint lines?

Dave Pellegrene has a writeup somewhere on his method, it is somewhat involved. I have had good luck with a somewhat less involved approach. Both involve extra router work. Some quick searching should find it for you.

[PWR RYD]

Re: Has anyone mastered hiding joint lines?

Dave has a very nice blog about his "trench" method of hiding MDF seams. Do a seach for him or one of his designs and you will see a link in signature. Search for "dragsters".

[ROTECH]

Re: Has anyone mastered hiding joint lines?

Filler primer.

If your woodwork is sound, 2-3 coats of filler primer and extra detail in your sanding. Done.

[dthomas]

Re: Has anyone mastered hiding joint lines?

Hi everyone,

For the past week I have been testing, testing, and more testing ways to hide joint lines on my mdf build. Eventually I am going for the "piano black" look. So far I have tried wood glue, bondo, spackling and wood filler with no luck. The closest I came was with wood glue, which actually hid the lines but created new lines on each side of the glue. Today I picked up a can of filler primer so I will try that tonight but I was curious if anyone had a better way to hide these things.

Thanks

Good high build primer does the trick and 220 grit and a 300 grit wet sand usually cleans up all very nicely. Rustoleum has a good primer you can get at Lowes or HD by the gallon if you have an HVLP gun. If no spray gun get the rustoleum rusty metal primer it is pretty thick. If you need bondo you probably need to re-square your table saw.

[diy speaker guy]

Re: Has anyone mastered hiding joint lines?

There's a bunch of different ways.
Use materials that don't expand so much with moisture. Mdf is a sponge, it will swell much more quickly and dramatically than any other flat wood panel product I know of (but all wood is going to expand and contract at least a bit no matter what you do).
Miter cuts.
Waterproof layer(s). You'll need to coat the ENTIRE surface, inside and out with several coats of toxic chemicals. There's not many finishes that are truly waterproof though, humidity will go right through most finishes people will recommend, the wood glue you tested, bondo, zinsser (sp?) primer, none of these often recommended methods work. The only thing glue will do is make painting easier. I'm pretty sure dry glue will dissolve completely in water.
Use the seams as accent marks in the aesthetic design.
Orient all panels in the same direction, like translam construction. If they all go the same way all the layers will all expand and contract the same.
Climate and humidity controlled environment.

That's just a few different ways.

[johnnyrichards]

Re: Has anyone mastered hiding joint lines?

There's a bunch of different ways.
Use materials that don't expand so much with moisture. Mdf is a sponge, it will swell much more quickly and dramatically than any other flat wood panel product I know of.
Miter cuts.
Waterproof layer(s). You'll need to coat the ENTIRE surface, inside and out with several coats of toxic chemicals. There's not many finishes that are truly waterproof though, humidity will go right through most finishes people will recommend.
Use the seams as accent marks in the aesthetic design.
Orient all panels in the same direction, like translam construction. If they all go the same way all the layers will all expand and contract the same.
Climate controlled environment.

That's just a few different ways.

Seam lines are not from expansion and contraction, even on MDF. I have speakers in piano black that are eight years old, with nary a seam showing. Little bit of filler, primer and enamel are all that is used on it. They have spent time in storage in summer and winter, suffering temperature swings from -20 to 115F (hotter in the shed). All 3/4" MDF, all still seamless. Humidity will not go through any properly cured paint, and that includes the cheapest rattle cans out there. Even with the port not sealed off, these old speaks are still seam free.

Seams are from not paying attention to the woodworking and not enough patience. Exactly like bodywork on a car, what you do underneath determines what it looks like on top.

[diy speaker guy]

Re: Has anyone mastered hiding joint lines?

This is contrary to everything I've ever heard on the subject, including from the wood pros on this site. I see that seams pop out even with no finish applied at all (actually it's hard to see but easy to feel). But I'll consider this viewpoint if you can explain why these lines typically show up months after a finish is applied, and usually in times of humidity swings.

You have to remember that you do a LOT more work on your cabs than most people, I've seen the pics. You probably have a lot more coats of paint (and primer) than most people, in fact I've read your method. That will make a big difference and I'd be willing to say that with a couple dozens coats of primer and paint you will be safe from most humidity events. I dare you to put it in the bathtub for a couple weeks though, unless it's protected with shellac or similar chemicals. (IIRC shellac is the waterproof one.)

Until Johnny convinces me otherwise, this is driven by humidity. Even temperature swings can swell panels (although it's got to be a big swing to make a noticable difference). I once sanded a box that had been sitting out in the sun all day, it was pretty hot. When it cooled down a couple hours later, all the seams popped right out.

[ROTECH]

Re: Has anyone mastered hiding joint lines?

Ok, my previous response was a best case scario, and it often works for me.
If you feel that you would like a little more assurance than just the filler primer, I offered this advice to somebody here and they said it worked the nutz.

Do the glue trick like you did, painting on a water/glue mix until dry and hard. Wait an extra day or two, then run your router and flush trim bit over all the glued joints again. Watch in amazement as you see how much comes off due to swelling from the water/glue.
Then you can seal again with water and glue, or you can skip right to the filler primer. If you are really concerned, you already have tried Bondo.... and I cannot see why that wouldnt have given you perfect results as it is failsafe when done correctly ( and bondo is very forgiving).
The part about Bondo that scared people, is when you prime over bondo patches, you can see it through the primer.. this makes most worried that you will see it through the paint but with the filler primer ( or any primer) you will be able to cover it without worry.

[bobbarkto]

Re: Has anyone mastered hiding joint lines?

Every new generation seems to forget about the ways wood reacts to its envionment, and the consequences.
Woodworkers have seemingly been rediscovering this bit of knowledge for centuries.

Any wood product expands and contracts with changes in moisture content.
No practical finish will stop it.
Some will hinder it, sometimes enough to make a diiference.
This is indisputible.

Check with the Forrest Products Labs at UW Madison. They have some docs online with their test results and (gasp) science behind the conclusions that might be convincing. Or you can consider the hard learned lessons of hundreds of generations of woodworkers and the practical experiences they share.
Or better still, do some testing and observe. Just be thorough, vary the moisture content in a measured amount. Weigh your samples carefully, examine them visually with consideration and see what happens.

I'll also mention Dave Pellegrene. In his sig line is a link to his trench and fill thread here on TT that describes his method and some testing he did. This looks like a fairly practical approach that gets better than average results.
It appears that by eliminating the glue line at the surface the bondo filled trench extends the distance and moderates the amount of shrink and swell induced rise and fall of the mdf around the joint line. I suspect the somewhat soft, flexible nature of bondo compared to most adhesives is also a contributing factor.

Some background.
Moisture vapor mollecules are very small. Much smaller than the spaces between the molecular chains and crystaline structure in any practical finish.
Look at GoreTex and similar fabrics that allow moisture (water vapor, humidity) to pass through while keeping liquid water and wind out. The fabric is tight enough to stop air and water, but not water vapor.
Wood (and products like mdf that are made from wood fibers), uniquely has an afinity for water and water vapor. The very nature of the cellular structure acts somewhat like a sponge or siphon that draws moisture from the surroundings. IT is after all the natural function of these cells to draw water from the soil and up into the branches. ;)

Finishes can be and are rated for for their ability to resist water vapor penetration. This is called the MEE or Moisture Exclusion Effectiveness
Pigmented finishes fair best, clears are less effective.
Most oil based paints have a long term (over 14 days) MEE value (moisture excluding effectiveness) at best ~60% with 3 or more coats.
Clear varnishes fair worse.
Most primers are ~20-40% or so, some are even negative due to fillers used to promote sanding and filling.
Lacquers range from the 20's to 50's to a little more if a catalyzed lacquer is used or if a vinyl sealer is used as a sealer (neded to pass KCMA tests for moisture exclusion btw).
Polyesters fair badly at mee without a good sealer like vinyl or epoxy.
Some 2 part urethanes approach the upper 80's.
Shellac is good to almost great at ~40% for plain shellac to ~70% for some pigmented shellac (eg: Killz or BIN).
Most latex/acrylic finishes are poor and are often used when more free passage of water vapor is desired. Especially in exterior finishes. This is intentional to help reduce blistering from trapped water vapor pressure and the eventual film failure it causes.

Most 2 part epoxy finishes are better than the best 2 part urethanes and are the best practical ME finish. That is one reason it is used by good auto refinishers as their first primer coat, it seals the metal from Moisture (water vapor) that passes through the rest of the multiple layers of primers, sealers and paints that are applied over it. It's also preferred by most boat builders as a laminating resin where it excells at keeping water out of their fiberglass hulls (ever see the water blisters in a polyester/fiberglass hull? :D)

Parafin wax (dipped, six layers iir) will be ~100%, so will some asphalt compounds and plastic encapsulation. IOW, nothing too practical.


As to seams showing.. Much depends on personal expectatons and the quality of the finish and the observers tolerance for defects.
A very good high gloss finish, in a dark color, will show the seams without some attention beyond carefull joinery, sanding and finishing. Unless the relative humidity never varies for any length of time from the time of finishing.

In my testing I observed that;
Most adhesives do not expand or contract at the same rate as the mdf. They remain relatively stable while the mdf increases or decreases in size around the glue adhered joint.
As mdf shrinks and expands with changes in moisture content (from moisture in the air) the glue line remains fixed relative to the movement of the flat panel. This leaves a small dip or a slight bulge at the glue line. This defective appearance extends outward some distance in at first a sharply decreasing then more gradual slope. The amount the slope extended varied but was inconsequential compared to the sharp rise or fall very close to the glue line.
Further, the difference in hardness between the adhesive and the mdf caused some resulting irregularity in the surface immediately around and on the glue line after sanding. This was only alleviated by surface coating the area and further sanding. This might be a contributing factor but I have not researched it further. It was a well known phenomenon to me beore I started my observations.

Here is an image, taken from 2 directions, of the corner of a cabinet I've had sitting around a long time (please excuse the dust!).



After thorough sanding to as level as possible a coat of shellacwas applied on the ends/edges, fine sanded and followed by 1 coat of vinyl sealer, then 3-4 coats of lacquer primer/surfacer sanded level, another coat of primer lightly sanded and then many coats of gloss black lacquer, which was sanded and polished to a high gloss. The finish was applied during a period of relaively low humidity. It showed no seams when in a high gloss state. Shortly after finishing the weather turned more humid and the seams showed.
I began to sand back, hencce the dull surface, and stopped when the seam almost disappeared. It always remains visible due the differing gloss level relative to the rest of the surface.

You can probably see the bright glossy line in the top image, along the right edge extending up and slightly left toward the top of the image. There is an identical seam along the edge in the foreground that is invisible in this image to my eye.
At most other angles the visible seam is invisble as in the lower image.

I have some veneered enclosures that show a bit of change at the joint line in varying weather. Even with glue, veneer and multiple coats of finish the moisture still gets in.

Light and brighter colors, low sheen and defects like orange peel, wrinkles, scratches, and others all can hinder the visual appearance of these ugly seems. But look carefully, in the right light you should be able to find them.

Hope this is somewhat helpful.

[diy speaker guy]

Re: Has anyone mastered hiding joint lines?

That's the best post on the subject I've ever seen but I can't see your picture.

[bobbarkto]

Re: Has anyone mastered hiding joint lines?

I think I fixed it...

That's the best post on the subject I've ever seen but I can't see your picture.

[diy speaker guy]

Re: Has anyone mastered hiding joint lines?

Now I see it.

A list of your favorite product recommendations would be nice too, for each of the different types of product. If you feel like it.

[takitaj]

Re: Has anyone mastered hiding joint lines?

Not withstanding all the environmental issues, the best stuff I've found so far is automotive glazing compound. Usually find it at a hardware or automotive store. It comes in a squeeze tube like toothpaste and is fairly inexpensive ($5-7 a tube). It's the last coat used after a bondo job and is very fine. Spreads nice and sands easy. I use it on the edges too, just don't sand through it. I also use flat black and apply more than a few coats with some light sanding or steel wool between every couple coats and before the final coat. Then poly, etc..

TJ