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What Filler???

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PolymorphicMark View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PolymorphicMark Quote  Post ReplyReply Direct Link To This Post Posted: 10 March 2017 at 7:22pm
Originally posted by SouthwestCNC SouthwestCNC wrote:

No matter the filler, filler doesn't swell with the ply when paint is applied. Without applying filler to the entire cab filler will always be visible. Plug the counterbored screw holes with wooden plugs is the only way to make invisble with paint.


Thanks for that tip :)

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bee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bee Quote  Post ReplyReply Direct Link To This Post Posted: 10 March 2017 at 11:41pm
so much miss information hear guys....
 
screws are used to hold panels in place till the glue drys, they add no strength to cab.
 
glue and clamps are a must, and clamps should remain holding panels in place till glue is firmly dry.
 
adding more screws will not make a box stronger, the weekest point to a box is the cheap ass pva glue people use. best glue for birch ply is commonly known as yellow glue.
 
look for tight bond 2 or tight bond 3, only difference is drying time of glue.
 
best filler, defo not wood filler for speaker boxes, use a good car body filler like upol, available from most car shops (Halfords)
 
wood filler is too soft for speaker boxes and can fall out due to vibrations and moving boxes about. (in and out vans stacking ect)
 
upol also is easy to sand and most paints stick to it.
 
wood only swells if your using water based paint and cheap wood, to stop your cabs swelling using water based paint, put a very thin coat on, let it dry fully, that way it creates a barrier and paint coats added after wont swell the wood. too much paint in one go can also swell the wood.
 
if correctly done with wood filler your filled area wont show when painted, if the paint is too thin then your not coating the cabs with a thick enough coat, again use more coats...
 
best method to join panels on a speaker box is the good old simple rebate, no need for dowels or rabbit's or dominoes or any of the other methods. simple rebate gives the maximum glue to surface area.. and also acts as a guide to aligning your panels.
 
but that said what do I know.....


Edited by bee - 10 March 2017 at 11:46pm
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SouthwestCNC View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SouthwestCNC Quote  Post ReplyReply Direct Link To This Post Posted: 11 March 2017 at 12:14pm
I only use quality Birch ply bee, and it doesn't require any paint to swell. Humid conditions are all it needs but all wood is unstable is will expand and contract continuously. When have you had an 18mm sheet actually measure 18mm.

You would need to ensure the humidity is stable in your workshop and the paint applied has no swelling effect on the wood as celulose based paints will still swell or shrink wood (polyurethane maybe an exception), and the cab is completely sealed after the coating, But sanding filler in wood also is not an ideal situation as they do not sand at even rates.

I personally now try to avoid filler at all costs.







Edited by SouthwestCNC - 11 March 2017 at 12:28pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nickyburnell Quote  Post ReplyReply Direct Link To This Post Posted: 12 March 2017 at 11:25am
P38 shrinks in time, not the wood, does it on metal too. Didn't used to, does today. Something changed?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 12 March 2017 at 3:05pm
well there seem to be a lot of different views here. still contend that screws will only add strength if the joint is poorly glued or not a suitable joint for plywood e.g. one that sticks one piece of wood to the surface of one laminate on the other piece of wood and relies on the strength of the glue between laminates.

no one has mentioned dry joints. if you clamp/screw together a joint too tightly you will push most of the glue out and not leave enough to soak into the surfaces and glue the pieces properly. whether using PVA or PU I always apply the glue to both surfaces and leave it to soak in for several minutes. then screw together just enough that the joint closes up and glue bulges out the sides along the whole length. as the glue continues to soak in and cure it will pull the pieces a few micrometres closer together if necessary. If done properly the glue is stronger than the wood.

butt and rebate joints are not great for plywood because the right angles follow the structural weakness of the ply layers. they are convenient though because they are easy to machine and, as bee said, easy to align and keep everything square.

southwestcnc is right, best way to do screwholes is with wooden plugs. after this (half decent) wood filler - the people who make this aren't morons. it's formulated to stick to wood, to sand at the same rate as wood, to take paint the same as wood, and to expand with heat/moisture the same as wood. if P38 was that great for filling wood you will find it different tins with 'professional wood filler' printed on it. the only reason I could see for maybe using it would be if your screws aren't countersunk very far so the filler is mostly stuck to metal rather than wood. deep countersinks with all the dust blow out have never given me any problem with filler coming out.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bee Quote  Post ReplyReply Direct Link To This Post Posted: 12 March 2017 at 3:11pm
ive seen water based paints, offer no help when it comes to temperature change, and cabs swell shrink, but never bad enough for filler to fall out, even on cabs 30 years old. If your seeing movement that bad, I would question the storage methods of my wood supplier, also all sheets of birch should be left to climatized for 2 weeks to your workshop temp. once painted up, there should be very little change, as that's what the paint is supposed to help do as well as protect. an 18mm sheet can vary in size by around 0.5 of a mm, any more than that and again, the wood goes back to wood yard.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bee Quote  Post ReplyReply Direct Link To This Post Posted: 12 March 2017 at 3:15pm
ive not used p36, I just use the upol easy fill version. its the choice of brands like void, turbo, ass, martin to name afew, so defo good enough for me... never had it fall out yet..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote snowflake Quote  Post ReplyReply Direct Link To This Post Posted: 12 March 2017 at 3:53pm
probably overkill but waterproof PVA on the inside of the cabs makes them totally moisture proof.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nickyburnell Quote  Post ReplyReply Direct Link To This Post Posted: 12 March 2017 at 4:11pm
Originally posted by snowflake snowflake wrote:

probably overkill but waterproof PVA on the inside of the cabs makes them totally moisture proof.


Always with MDF


Edited by nickyburnell - 12 March 2017 at 4:11pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bee Quote  Post ReplyReply Direct Link To This Post Posted: 12 March 2017 at 5:58pm
100% with mdf, I also pva any cut edges once assembled pre painting. This helps stop the furring you get on edges.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 70,s hero Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 10:00am
Screws clamp the joints together and distribute any forces further in to the opposing planes of material effecticevly increasing the area of distibution of force should any lateral or compressive forces be applied.

To increase the contact area for the same purpose and alignment, rebating is better than butt joints,the grain really has no effect as the grain in ply is set upon each layer at 90 degrees in opposition. It is questionable if there is any appreciable soaking in of glue as the ply (grade bb Birch) is compressed greatly with glue at manufacturer making it very dense and the viscosity of PVA and bubble glue is a deterring factor.

I do not agree that a screw does not add to the strength of a corner joint, for sure, the glue does the majority of work in stabilization but screws will help as they help distribute forces.

The idea that glue is the only requirement for the strongest joint is true in say glue beams or ply which are manufactured with hydraulic presses, I am open to be convinced that a corner joint in say 18mm ply does not benefit from additional screws, that's my take on it, hoping to be convinced otherwise.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SouthwestCNC Quote  Post ReplyReply Direct Link To This Post Posted: 13 March 2017 at 10:15am
Theres certainly a balance, adding excessive amounts of screws will compromise the structure as panels can then split through its thickness however I agree a minimal amount of well placed screws does increase joint strength over just being glued.
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