Wet sanding epoxy when attaching scales

E

elementfe

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Aaron has an excellent thread discussing the strength of the bond between tangs and scales and this came up- I thought I'd pass this along in a thread of its own.

Looking at the table for bond strengths for West Systems epoxy, I noticed that it listed double the bond strength to steel if you "abrade through wet epoxy."

The way the West Systems techies explained it to me is this: The reason stainless steel is stainless is that it reacts with the air and forms a protective oxide layer. This happens very quickly- if you've done any aluminum welding, you know that this can be an issue even on a freshly ground, clean surface, and part of the reason for using AC high frequency is that the polarity has to be reversed many times a second to to clean the metal as you weld.

We think of aluminum and steel as very stable materials in terms of corrosion, but that's only because it corrodes REALLY FAST and then it's done, and the layer of oxide prevents further reaction.

The purpose of wet sanding is to bond to steel instead of the oxides on top of the steel.

So by this way of thinking, sandblasting helps, since it's more surface area, but you're still gluing to a thin oxide layer rather than the steel itself.
They recommended wire brushing, but I wasn't sure that it was suitable for a hardened, wear resistant alloy, so I chose to use a chunk of worn ceramic belt, since I see no evidence that it's shedding crud that would compromise the joint.

I'm trusting that one of the really knowledgeable people will correct this if I've made any assumptions that don't match how steels actually behave!

Hope this is helpful.

Andy
 
Wet sanding any resin does three things:
One, it cleans the surface, removing any contaminates and oxides...just as you opined.
Two, it adds some of this removed surface material and a small amount of abrasive grit to the resin. A "filled" resin is stronger than plain resin. This is because the interstitial distance between these particles is much smaller than the distance between the tang and scale. This shorter bond distance is stronger.
Three, it greatly increases the mechanical contact with the resin and the material, literally scrubbing it into the surface pits and ridges ( look at a blade tang surface under 100X magnification). It also provides many new and sharp peaks and valleys to increase the bonding surface area.

None of these gains will be significant in 99% of all knife construction, but if you want the very best bond strength, wet sanding the resin will increase it.
 
When I began making small general purpose kitchen knives, the rivetless assembly was kind of a Grail for me- I really liked the appearance of smooth wood without visible fasteners.
My tastes have changed since then, now I feel that a rivet of some kind gives it a "visual logic" that makes it look like a better product.

Still, I have to wonder...West Systems claims DOUBLE the bond strength using wet sanding, according to this table:
http://www.westsystem.com/ss/g-flex-epoxy-adhesion-data/

(They don't list the kind of steels that we use on that table, but based on my conversations with them the same standards apply)

Is this an insignificant factor for 99% of all knife construction because the epoxy is seen as only a sealer? In that case, we'd be better off using one of the sooper dooper marine sealants like 5200 and just trusting mechanical connections for structural integrity (I think this would be perfectly fine).
Why use epoxy at all, if bond strength is insignificant?

Do I understand your post correctly?
 
What I am saying is that in most all knife handle construction, the epoxy bond strength is sufficient without any wet sanding. If I was building a boat, I would want every extra pound of strength. In attaching the handle on a knife, the extra strength is insignificant.

Sort of like saying, "A grade 8 bolt is twice as strong as a grade 2 bolt." That is true, and especially important when building a sky-scraper...... but if you are bolting a license plate on a car, the extra strength is insignificant.
 
I would venture this guess. regularly applied epoxy and rivets.and bolts are already extremely strong. so doubling the strength of the epoxy is not making much of a change but the need for the epoxy is there to support the mechanical hardware and also serve as a sealant.
 
I've been thinking about this for a couple days now, since Aaron's thread. It's certainly worth discussing.

For whatever it's worth, I've done rehandle jobs on a couple semi-custom/factory knives (the brand of which shall remain nameless... but I bet some of you can guess) where the scales either literally fell right off when I drilled the bolts out, or came free with a tap. You could tell there was some sort of epoxy on there, but it was either junk or hurriedly prepared or squeezed-out too much or who-knows-what... it wasn't doing any good at all.

The point is, the handles appeared to be in perfect working order before I started messing with them. You never would have known the epoxy failed until I removed the Corby bolts... unless maybe moisture seeped under the scales and the tang eventually rotted off. That could take years or decades. Frankly, they might as well not have even bothered with whatever epoxy they used and saved a couple dollars.

I'm not saying you're wrong by any means, and if I was ever convinced to use only epoxy and no mechanical fastener (extremely doubtful), I would certainly give wet-sanding a try. But my main interest in epoxy is indeed as a sealant. I neither trust it nor need it for strength... any strength it adds is a bonus.

I would rather use bolts and no epoxy, than epoxy and no bolts, any day.
 
james terrio said:
I would rather use bolts and no epoxy, than epoxy and no bolts, any day.
Click to expand...

+1, Except for the several instances where I have needed to use hidden pins to achieve a certain look. In those, a lopsided placement of mechanical fasteners was used (either peened pins or flared tubes.) I have done the wet sanding in since I first read it (can't remember if it was elsewhere or posted here.) In the latest instance on a larger chopper, I watched as the new owner used the handle as a mallet. No separation of any kind. Whether or not it would have been the same without the wet sanding in isn't known, but I figure it couldn't hurt. There were two flared tubes and five threaded hidden pins with keyholed scales as well as the epoxy though...

Aarons video had me take a second look at the strength of the corbys though, and given the large range of sizes I think I will start replacing pin use with those...
 
I have commented many times that the main reason for epoxy failure is poor prep. I learned from a boat and furniture builder when I was in high school. I have projects made back then, think 1987 or so that are still together, even with the basic lepage 5 min epoxy. Surfaces were clean and prepped, and the glue was mixed the right length of time. Every single failure I have had has been on a project where I cut corners on prep. I have seen items break beside the glue joint, suggesting the glue is stronger than the parent material.
 
So when you say wet sanding the resin/the wet is the resin.Just tryin to wrap my head around it.You would sand or hollow grind the tang first and clean. Then apply resin and scrub it in with course grit paper.This is more of a ? than a statement.
Eddie
 
Eddie- Yes.
Stacy- Thanks for the clarification.

I'll say again that there are far better sealers that are far less trouble, if all you're doing is sealing the joint.

For now, I'm liking the idea of having a sealer that can be trusted to also hold the scales firmly in place, and since G Flex has a bit of give to it, it should be able to cope with at least some swelling/shrinkage of the scales due to heat and moisture.

...but hey- the more I do this the less I think I know, so all opinions are subject to change over time :)
 
If I want the scales to stay on for a really long time I use Loveless bolts or Corby bolts and any of the epoxies I have on hand. If I want to use beautiful mosaic pins and I'm worried about the epoxy grip then I cut grooves along the tang in parallel fashion and also cut grooves at a right angle to the first grooves. I use a Dremel tool with fiberglass reinforced cutting blade to make shallow grooves 50 or 60 thousands of an inch deep and the first grooves I make I tilt the Dremel tool at a 60 degree angle to the surface of the tang . I then make another parallel cut at 60 degrees at the bottom of the tang so that the first two cuts are angled away from each other. When the epoxy fills these top and bottom groves it forms a claw of epoxy . These grooves help the epoxy not shear off from a side blow to the scales. I also drill a couple 1/4 inch holes in the tang to give the epoxy a rivet shape through the tang to help hold. Maybe this isn't necessary but I have made over 300 knives and have not had one returned because the scales came loose. For hard use knives I like Loveless bolts and grooves. Lanyard holes also provide more stability because it is like having 3 pins instead of 2 and the lanyard hole keeps the other 2 pins from moving. I clean my blades with Acetone before epoxy. I can't think of anything else to strengthen the bond but maybe some of you other knife makers have a secret for us.
 
chemistguy said:
wet sanding sounds messy.
Click to expand...

They make these neat things called nitrile gloves...

Dude, I know this is Stacy's thing, and not trying to step on toes but since you have disallowed PM's...: Have more to say than 'buy an ESSE (sp) 4' or 'sounds messy'

-Eric
 
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