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Martice-


Thanks for the Pics, is always interesting to see how stuff is built. I assume these are acoustic suspension in design, thereby the importance of a truly sealed cabinet. The pics don't show any kind of interior sealant (chaulking) in the corners, where you planning on that? One way to test a cabinet for air leaks is to plug the speaker holes, install a schrader value and pump it up with air pressure to check for leaks. (caution! not to much pressure, you don't want them to explode).


It is almost a waste that he went to the trouble to miter all the outside corners, and then just ended up doing a 1/2"radius (or so) round over, ie kinda defeats doing the miter joints. (Picture if these were hardwood veneered ply, then the miter joints would allow for a better grain match - wrap-around). Heavy radiused corners are very good especially for the front baffle (diffractions).


Looks from the picture that everything is butt jointed, did he he use bisquits (these are like little mini splines)? Also looks like the glue used was a standard PVA or alphatic resin (carpenters glue).


Tip for the future, if you are going to do it again, try to get them to use marine epoxy with a silica type thickening agent. (Makes for a less fragile joint).


Is that a full 1" thick MDF he used? All the way.


Are you going to coat / seal the inside (sanding sealer etc.)?


The routered outside corners are going to require extra effort to seal/prime the MDF pryor to painting. The interior core of MDF is more porous, so it tends to soak up the paint like a sponge. Several coats of high solids undercoater (sanding in between coats) eventually seals these so you can get a perfect paint job.


Regards, Bruce
 

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Discussion Starter · #3 ·
Hi Bruce. Thanks for the response.


"The pics don't show any kind of interior sealant (chaulking) in the corners, where you planning on that? One way to test a cabinet for air leaks is to plug the speaker holes, install a schrader value and pump it up with air pressure to check for leaks. (caution! not to much pressure, you don't want them to explode)."


As the links have stated The cabinets were done by Brian Bunge of Rutledge Audio Designs. I specifically remember him telling me while constructing my cabinets that he wanted to make sure that he had a good seal on the cabinets. How he came to this conclusion I don't know but I did forward your post to him so he should be along shortly to answer the more technical questions you may have.


"Are you going to coat / seal the inside (sanding sealer etc.)?


The routered outside corners are going to require extra effort to seal/prime the MDF pryor to painting. The interior core of MDF is more porous, so it tends to soak up the paint like a sponge. Several coats of high solids undercoater (sanding in between coats) eventually seals these so you can get a perfect paint job. "


When you say "several coats", how many would you suggest? I am meeting with the auto body mechanic who is going to finish my cabinets so I want to make sure that I have all of the bases covered.


Thanks for your reply.




"
 

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Martice-


The number of seal/undercoater coats will be a function of several things.


-The main point being which type (brand and grade) of MDF was used. I almost always use a 'super-refined MDF' normally Plum Creek II (mfgr), as it tends to be easier to seal and finish.


- lightweight Bondo (or equivalent) also does a pretty good job of sealing the routered edges. Glue size can work too. (thinned glue smeared on).


- The type of top coat finishing may have some bearing also. For example nitrocellulose lacquer (furniture) is generally thinner bodied and would require more coats to hide any imperfections. Whereas, heavy bodied Catalyzed polyurethane enamel would take fewer coats. I can not speak so much to the 'Auto paint' as I don't know as much about it, relative to this.


- If you are going to end up with a high gloss (piano type) finish, even the smallest imperfection can shine through. Typically an edge that wasn't sealed properly will show some traces of porousness (little puck marks, if you will).


- Generally speaking 3 coats of primer should work in most cases. Sanding smooth in between.


A little extra effort at this stage will pay-off in the log run, especially if your auto paint guy has not tried finishing MDF before. There is basically no soak-in on metal.



Regards, Bruce
 

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Discussion Starter · #5 ·
Thanks a million for your input Bruce. I have printed your response and will take your input with me whenI meet the mechanic. Depending on the out come of my next meeting with the mechanic I will make the final decision as to what needs to be done to ensure that I have the finish I envison.


Thanks and I'll post the finished photos soon.
 

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I have a few questions for Brucer


Do you think that using bisquits with butt joints is the way to go? Is there a better way to join the sides?


Is it better to use a glue like Gorilla Glue than a regular carpenters glue?


What is the benefit of sealing the inside of the box? Would you do this before or after you chalk?


Thanks! I'm getting ready to build my own (either AV1+ or Audax HT) and I would like to do the best job I can.
 

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Brucer,


I built the cabinets for Martice. The reason I used mitered joints was to minimize the seams that would need to be sealed on the enclosure. If I had used regular 90 degree butt joints there would have been 4 seams all the way around the top and bottom and two seams down each side. In my experience, there would be much more work involved using 90 degree butt joints.


Also, I used a 3/4" roundover bit on all edges. And yes, I used Titebond yellow carpenter's glue as it provides a very strong bond. As you probably know already, the glue joint itself is stronger than the MDF. Also, these are vented enclosures; not sealed. But I used liberal amounts of glue and didn't feel it was necessary to add anything additional to properly seal the joints internally. All material was 3/4" MDF.


Thanks for the tip on the marine epoxy. I may try that in the future.


Brian Bunge
 

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Quote:
Originally posted by Clark
I have a few questions for Brucer


Do you think that using bisquits with butt joints is the way to go? Is there a better way to join the sides?


Is it better to use a glue like Gorilla Glue than a regular carpenters glue?


What is the benefit of sealing the inside of the box? Would you do this before or after you chalk?


Thanks! I'm getting ready to build my own (either AV1+ or Audax HT) and I would like to do the best job I can.
Hi Clark-

Joints


Butt joints by themselves are the easiest but also the weakest type of wood joint. They are greatly improved by using bisquits, dowels or splines. Bisquits are the easiest and fastest and most foregiving of the above.


Dado joints are even better yet. As well as finger joints or dovetails. But there are certain applications where one joint may be more appropriate over another.


For example, a dado joint to attach shelves (or braces) not only provides more glue surface area, but is holds the shelf or brace captive. A rabbet joint is like a dado except it is out at the edge.


A miter joint is similar to a butt joint, but does provide more surface area. A miter joints strength can be improved using splines or bisquits. There is also a joint called a locking miter joint, where the machining provides a 'built-in' spline of sorts.


As far a creating a sealed joint, the more 'contorted' the joint line, the better. There are various others joints that may not apply to this discussion.


Glue


Gorilla glue is a brand of polyurethane glue. There are various other brands as well. The advantages of polyurethane glues deal mainly with two things: waterproofness and gap filling. It is much more expensive. It can be a very strong glue if it is applied right. 'Poly' glues need moisture to cure. If gluing MDF (or just about anything else) you need to dampen the surfaces prior to application. The gap filling properties come from when the glue starts to setup it actually swells, (kinda like those cans of spray foam that you use to seal around window leaks, etc) although not that much swelling. Since it swells(a little) it does require that you leave it clamped up during this time. Epoxy, if properly used, is stronger, waterproof and actually more flexible.

Sealing inside box


MDF is a porous material. Porosity varies widely betweens brands and /or grades. (I can pull a vacumn through it, with varying results). By sealing the surface it limits two things: Air and moisture/vapor exchange. Spraying the inside of any small box is also a 'paint in the butt', so it may be easier to pre-seal the surfaces before assembly. I would chaulk it afterwards, regardless of how you applied the coating to the inside. I have heard people say to be careful what kind of chaulking you use. (usually refering to silicone) The outgassing of the solvents is feared to attack driver components. Latex is safer, although if the boxes were built and allowed to cure for up to a month, then it probably would not be a concern.



Most of these comments are relative to plywoods, ie. MDF or regular, hardwoods need more discussion.


One final note: I hear a lot of people talk about screwing boxes together. I consider brad nails and screws as merely temporary clamps until the glue cures. There is virtually no holding power when fastening into the edge of plywoods, or even hardwoods 'endgrain'. If you want to know a better way - I can tell you.


Are you building paint grade boxes? or veneered? or other?


Good Luck - if you, need 'verbal' help, let me know.


Regards, Bruce
 

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Quote:
Originally posted by Brian Bunge
Brucer,


I built the cabinets for Martice. The reason I used mitered joints was to minimize the seams that would need to be sealed on the enclosure. If I had used regular 90 degree butt joints there would have been 4 seams all the way around the top and bottom and two seams down each side. In my experience, there would be much more work involved using 90 degree butt joints.


Also, I used a 3/4" roundover bit on all edges. And yes, I used Titebond yellow carpenter's glue as it provides a very strong bond. As you probably know already, the glue joint itself is stronger than the MDF. Also, these are vented enclosures; not sealed. But I used liberal amounts of glue and didn't feel it was necessary to add anything additional to properly seal the joints internally. All material was 3/4" MDF.


Thanks for the tip on the marine epoxy. I may try that in the future.


Brian Bunge
Thanks Brian-


Sorry, I assumed sealed enclosures.


A Vented box is less significant. I think there are 12 corners, any way you count them. I just feel miter joints can be alot more troublesome during construction and assembly. But that's fine.


Nice job - 'Sounds' (pun intended) like Martice really likes the results.


Regards, Bruce
 

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Brucer,


It's not the number of corners, but the fact that using a butt joint leaves you with trying to hide the seam between the face of one board and the end of another. The cut ends of the MDF are much more porous, as you already know, and the seams have been much harder to hide in past experiences.


I agree that miters can be more troublesome. On most veneered enclosures we will be using butt joints along with glue and brads for speed of assembly. Also, this just happens to be the way that Danny Richie prefers the raw MDF enclosures built. Since my work is a direct representation of him, I'm inclined to oblige! :)


Brian Bunge
 

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Quote:
Good Luck - if you, need 'verbal' help, let me know.
Thanks, I will. I've been doing a lot of reading and am starting to get the hang of it. The reason I asked about the joints is that I am to a big fan of butt joint (it sounds like a bad joke), but I haven't come up with a much better solution. I am planning on milling small rabbits and dados (1/8th inch) to strengthen the joints.


In what form does the marine epoxy come in. Can you recommend any brand names.
 

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Brian-


1/2 dozen vs. 6 another once it's routered. We definitely agree about dealing with edges of MDF, That's why I recommended to Martice to take the extra effort to seal/primer/undercoat the routered edges.


Yes and I too will oblige my customers wishes.


BTW, One other advantage of using epoxy is to avoid glue creep at the joint lines. If in the future you decide to try it, be sure to get some silica filler to thicken it up. I use West System Epoxy, but ther are others just as good.


Do you do your own finishing? If so, if you like that black textured look, that is usually done with catalyzed poly. enamel, spray a smooth coat first then 'spit' the texture last. The stuff is very very tough!


Thanks again


Bruce
 

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Quote:
Originally posted by Clark


Thanks, I will. I've been doing a lot of reading and am starting to get the hang of it. The reason I asked about the joints is that I am to a big fan of butt joint (it sounds like a bad joke), but I haven't come up with a much better solution. I am planning on milling small rabbits and dados (1/8th inch) to strengthen the joints.


In what form does the marine epoxy come in. Can you recommend any brand names.
Oh! I think I just answered that in my reply to Brian. It's expensive stuff. I buy it in gallons, but it is available in smaller quantities. One thing I like about it is that you can get 'push spiggots' that dispence that exact ratio of hardener and resin. (Looks like the catsup and mustard dispensors at the ballpark). It works out to about 50 cents a squirt (approx 2 oz). So there is no waste.


If you have a West Marine Store near you - they carry it retail.


Regards, Bruce
 

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Brucer,


Yes, I do my own finishing, but do not have a spray rig set up yet. That's why Martice found someone local to him. I'd love a detailed description of how to do a textured finish for future reference though. I imagine it's cheaper than the textured stuff in the spray cans.


Brian Bunge
 

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Quote:
Originally posted by Brian Bunge
Brucer,


Yes, I do my own finishing, but do not have a spray rig set up yet. That's why Martice found someone local to him. I'd love a detailed description of how to do a textured finish for future reference though. I imagine it's cheaper than the textured stuff in the spray cans.


Brian Bunge
Well I don't know about 'cheap', but what I use is Polane made by Sherwin Williams (nothing they sell is 'cheap'). You need to have a spray gun that utilizes a pressure cup. Siphon feed won't work. Most of the Turbine driven HVLP's use a pressure cup, although the cheapies will not work as well. This stuff is up to 40-60% solids, heavy bodied. You have to mix the (Acid) catalyst with it and reducer. Smooth coats are like any spraying. The texture coat is achieved by reducing air spray pressure way down (but maintaining cup pressure), then you adjust to the size of droplet pattern you want. Polane can be force air dried, for a quicker cure.


I can not overstate how tough this stuff is. I tried sanding a cabinet done once to recoat - 'never again'.


Regards, Bruce
 

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Quote:
Originally posted by Clark
What is the silica that you mix in? Is it something that they sell at marine west? Thanks again for the tips.
The stuff is virtually lighter than air (fluff) and yes they sell it, too. It is amazing how such lightweight matter thickens the epoxy, but it does.


Bruce
 
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