The Garrett -Wade catalog has a PVA glue that it claims:
1) It fills gaps with strength
2) Squeeze out does not penetrate the wood and easily chips off when dried,
no need for scrapping.
If it actually does this I will certainly use it, but it seems like bull
when an obsure brand is radically superior to the successful brands.
Has anyone used it. (Hey, occassionally my joints have gaps...)
Well if it is like the Lee Valley 202GF, then yes. Fills gaps with
strength? I've never tested it, but it does indeed fill gaps. I like the
brown colour too. I have to shake it up real well before use as it the
solids settle out.
How about glues that hold metal pieces together. They don't penetrate the
metal. The problem is the strength of the glue bond to each piece and the
internal strength of the glue layer itself. A roughed up surface holds
better than a smooth one because there's more glue in the between pieces
volume, ain't it?
If it chips off easily, it doesn't seem to mean that the bond itself will be
weak. The stresses would be different, wouldn't they?
A roughed up surface does not necessarily mean a better bond. Have you ever
laid a pane of glass on top of another with water between the two? Almost
impossoble to pull apart because of the surface tension.
I don't want to be seen as a pedantic jerk by you chaps even though my
students often called me one but:
Glass sheets are held together by water between them because the water has
driven out the air and occupies the space between the two sheets. The
sheets themselves are then forced together (and against the water lamina) by
the air pressure that pushes them together at about 14 lb/sq in. The only
mediating pressure is the very slight air pressure on the thin lamina of
water between the sheets of glass that forces the water to push outward
against the sheets by pushing inward on the water. Surface tension has not
much to do with it unless there is an interactive attractive force in the
glass (or whatever) that pulls the water molecules toward the sheet but this
would be very slight in almost all cases of materials. Air pressure is the
force both holding the sheets together and, to an extremely small degree,
pushing them apart by pushing on the water lamina. If the glass sheets are
uniformly flat and measure, say 10"x10", each sheet is 100 sq in. The air
pressure produces 14 lb/sq in on each sheet so the total is [14 lb/sq in x
100 sq in/sheet x 2 sheets] - [the pressure on the water from the sides], or
2800 lb. (I ignored the very very slight pressure on the water lamina
because it close to zero). 2800 pounds is the amount of force needed to
separate the sheets if there's no other factor. We don't need that much
because when we slide the sheets over each other we reduce the amount of air
pressure push against them at the places where they are together. Finally,
by sliding the sheets enough, the holding pressure is reduced to an easy
force to apply.
Glues work differently and don't depend on air pressure to hold things
together. There are essentially two aspects, the adhesion of the glue to
the surfaces and the internal bonding strength between the molecules that
make up the glue itself. The solvent for the glue (water, alcohol,
whatever) is just a carrier. The glue dries by losing its carrier and (in
some cases) the chemical nature of the glue itself changes when that
happens. When it dries, it bonds to the things being glued and internally
inside the dab of glue between them. In the old days, both these bonds were
relatively weak and either the glue's internal bonds broke or the bond with
the material broke. Some glues produce bonds to materials that are stronger
than the material itself is internally AND the internal glue intermolecular
strength is also stronger.
Spaces in glued up joints produce weakness because theres no glue in the
spaces to form a bond.
Now you are showing signs that you might understand the point that I was
trying to make.
"A roughed up surface holds better than a smooth one because there's more
glue in the between pieces volume, ain't it?"
Roughed up surfaces often cause spaces in the glued up joint. A smooth
surface is Ideal for glue.
That is really debatable, mein Herr. It depends. If the glue can get into
the roughed up spaces, it will be stronger because there will be more glue
in the joint as a result of the increased surface area caused by the
roughening up. The gap causes a weakened area of the joint relative to the
rest of it because there's no glue there. This calls for scientific
investigation!! Roughed up vs. smooth glued surfaces. Aha, something I
If I do some sexperiments, would anyone want the results?
How would you design the set of experiments? I'll do them this weekend.
I believe the glue companies already have this data. IIRC their
recommendation is a smooth surface. Hence there are rip blades made for
glue line ripping that produce smooth surfaces. Jointers straighten a
board and produce smooth surfaces for gluing.
Thick glue in a joint is not desirable. It is usually an indicator of a
poor joint that does not fit well. Roughing up a surface has little value
for gluing as the surface produced from roughing up leaves loose and weak
fibers of wood.
I checked several maufacturers and other sites and found it about equally
divided between smooth and roughed up. Who knows? I did not find any raw
or reduced data but didn't look that hard.
First step - glue together two end-joint pieces. The ends are always
rougher than the sides. So the glue should hold better according to
the theory you are testing.
Some of you may have already done this... :-)
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I think it depends on how rough and what sort of glue. With most any
glue but epoxy the roughness must be small enough that you have a very
thin layer accross the interface. If you roughen the surface to the
point that the glue is filling gaps the result will be a weaker
joint than if you just planed the surfaces smooth and glued them
because the glue in the pockmarks will be weaker than the wood it
replaced. I dunno where you cross the line between giving the glue
tooth, and weakening the joint by requiring the glue to fill a gap.
I _think_ that a planed or scraped surface has the correct level
of roughness for most glues, and does not have as many loose fibers
or dust particles in the pores and does a surface roughened with
With epoxies, the glue in gaps is stronger than the wood and the
strength can be maximized by eliminating the wood altogether, though
the resulting furniture may leave something to be esthetically desired.
Forget the physics... wood fibers are held together by lignin and xylan,
woodworking glues mimic natural glues by bonding the wood fibers together.
That's why it isn't necessary for the glue to soak in and why a smoother
(planed) wood surface holds better than a rough (sawed/sanded) surface.
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