Official Luthiers Forum!

Hey folks,

Hoping to move a conversation from the "What's happening in your shop?" thread over to here.

Scraping surfaces before glue-up, specifically braces & bridges, makes sense. I do this for bridges and will consider doing it for braces although I'm a little skeptical of my ability to scrape a long brace like an X-brace leg without messing up the fit.

Here is what I do today:
- After finishing I level the finish on the instrument using 400 grit paper.
- Use a 30' radius dome workboard with 80 grit paper to sand in the initial shape on the bottom of the bridge.
- Use 1/2 sheet of 120 grit paper on the actual top to final fit the bottom of the bridge.
- Use the jig to put the rabbet around the bridge and test for fit with the top.
- Before glue-up use a fresh utility knife blade, wiped with naptha to get any oil off of it, to scrape just a whisper off the bottom of the bridge.
- On ebony bridges this leaves a quite shiny surface and that makes me nervous, so I also score the bottom of the bridge lightly in a crosshatch pattern with the same utility knife blade.
- Glue up with HHG

Any thoughts on scoring the bridge? Is it necessary? I tend to use maybe a little too much glue, but I don't stress because it is easy to clean up.

Note: I only mention the leveling of the finish because I want to do the final fit before I start sanding up the grits in case something goes wrong. I make sure to brush off the backside of the paper in case any grit is hanging out there.

Brad

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Good going Brad. It’s nice to have how-to topics in their own thread so they are easier to search.

How and why I do it -

I set braces with go bars in the radius dish so that is my goal for top shape.

I view the bridge as a big external top brace, and since up to that point I’ve built to the radius, I don’t have a problem with making the bridge to the radius and clamping it to the top. I don’t worry if the top has morphed a bit away from the original shape, as I expect the bridge “brace” to bring it back into compliance. Plus, I’m not sanding on the top potentially causing damage.

I wouldn’t cut hash marks in wood for HHG or TBond, just don’t think it necessary.

I rarely wipe with solvent as I suspect it will smear oil around what I just scraped clean.

I will roughen up wood to provide some tooth ahead of epoxy though.

Many ways to get er done. This is just where I’ve ended up.

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Brian R, Wood Mechanic
N8ZED

I feel the crosshatching is erroneous in function, especially with HHG.

HHG has no gap filling properties, so how would creating gaps with the cross hatching be functional?

On another forum someone posted a pic of a removed crosshatched bridge glued with HHG, and the glue in the crosshatches was crusty and opaque.

To me it looked like the glue dried without actually glueing anything, it was just ‘there’.

I would want to see some real data beyond anecdotal before I bought into any crosshatching schemes.

Just a clean, well fitted joint for me, with the bottom sanded fefreshed just before glue up…

Great question about scoring, I'll bite . Contemporary studies focus on the smoothness of the wood, and Planed Surfaces vs Sanded Surfaces as it relates to adhesion strength (mainly shear). I consider it presumptive that scoring is no longer believed to provide a better PVA glue joint with wood, as it is no longer (and for quite some time has not been) included in studies of PVA glue joint strength.

The referenced paper (file size too large: https://www.researchgate.net/publication/355131920) includes references to other papers and articles discussing the subject, without the topic of scoring. Damaged and misaligned fibers appear to be a main detractor to a strong shear joint.

Excerpt:
A rougher, but clean surface (without damaged fibers),
can provide enhanced capillary forces and expose more porous structure in the wood [14]. .

REFERENCE: DRITAN, AJDINA: The influence of sanding process on adhesive bonding of wood. Agricultural University of Tirana; Albanian j. agric. sci. 2021;20 (2): 35-39

I do my own adhesion testing as there are so many variables and so much "information" on the internet (materials, glue type, wood characteristics, size and orientation). I bought a force gauge on Ebay to legitimize my adhesions tests. You can do your own adhesion testing, subjectively, with popsicle sticks and glue (or spruce, or rosewood or cellulose binding on Walnut, ect.), if you're so inclined. I've learned much about adhesion since I started doing this, and I can test the effectiveness of the glues on my shelf too. I highly recommend adding this to your building arsenal.

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References in that paper:
1. Jaic M, Palia T, Djordjevic M: The impact of surface preparation of wood on the adhesion of certain types of coatings. Zastita Materijala 2014, 55 (2): 163–169.

2. Sabrina AM, Cláudio HS, Joana M, Mário RS: Bonding behavior of Eucalyptus benthamii wood to manufacture edge glued panels. Maderas Ciencia y Tecnología 2013, 15(1): 79–92.

3. Svetka K, Julie C, Roger EH: Evaluation of two surfacing methods on black spruce wood in relation to gluing performance. J. Wood Sci. 2013, 59: 185–194.

4. Lütkemeier B, Konnerth J, Militz H: Distinctive impact of processing techniques on bonding surfaces of acetylated and heat-treated beech woo and its relation to bonding strength. Forest Products Journal 2018, 68 (4): 372–382.

5. Gardner DJ, Blumentritt M, Wang L, Yildirim N: Adhesion Theories in Wood Adhesive Bonding. Rev. Adhes. Adhes. 2014, 2 (2): 127–172

6. Cheng E, Sun X: Effects of wood-surface roughness, adhesive viscosity and processing pressure on adhesion strength of protein adhesive. Journal of Adhesion Science and Technology 2006, 20 (9): 997–1017.

7. Forest Products Wood Handbook – Wood as an Engineering Material. US Department of Agriculture Forest Service; 2010

8. Creswell WJ: Research Design-Qualitative, Quantitative and Mixed Methods Approaches, Second Edition. SAGE Publications Thousand Oaks, London, New Delhi; 2003.

9. Kollmann FFP, Kuenzi EW, Stamm AJ: Principles of Wood Science and Technology – II –Wood Based Materials. Springer-Verlag Berlin, Heidelberg New-York; 1975.

10. Shields J: Adhesive handbook, 3rd edition. Butterworth & Co (Publishers) Ltd; 1984.

11. Ajdinaj D, Kortoçi M: Curriculum development under the focus of industry challenges. 4th Annual International Conference on Education and New Learning Technologies, Barcelona, 2-4 July, 2012.

12. Fellin M, Hernández RE, Negri M: Surface process effect on PVAc glued joints. 2nd International Scientific Conference on Hardwood Processing, Paris, 29 September, 2009.

13. EN 205: Test methods for wood adhesives for non-structural applications. Determination of tensile shear strength of lap-joints. CEN, Brussels; 1991.

14. ISO 13061-2: Physical and Mechanical Properties of Wood – Test Methods for Small Clear Wood Specimens: Part 2: Determination of density for physical and mechanical tests. International Organization for Standardization, CH-1211, Genève; 2014.

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Measure Twice,

Karl Borum

I scrape the bottom surfaces of all my braces before gluing using a fresh single edge razor blade. It only takes a few quick light passes to do the job and it doesn't affect the fit.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I also scrape both sides of all critical surfaces before gluing. I also use a single edge razor blade.

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Steve Smith
"Music is what feelings sound like"

I've been convinced in recent years that, with the way hot hide glue works, there is no point in intentionally roughing up or scoring the surfaces to be glued. In contrast, freshening up the surfaces to be glued by scraping right before gluing makes a lot of sense, and probably helps adhesion.

On the other hand, if the parts fit well, I wouldn't fret too much if such scraping is not practical in context, and only sanding can be done; I think mating the surfaces so that they fit together well matters more than whether the surfaces are scraped or sanded. Yes, scraped surfaces probably adhere better than sanded surfaces, but both are way better than any type (scraped or sanded) of ill-fitting joint. I'm thinking of things other than bridges here: plates to sides, braces to plates, etc.

We always scrap our bridges less than 15 minutes prior to gluing. Al Carruth had an article about this that he read and he's who turned me onto this scrapping idea nearly 20 years ago on this forum. I think this is called "joint energy" which reminds me it's nearly 4:20

Sanding leaves debris at the microscopic level, not a great thing for a joint that really needs to be near perfect, a bridge.

And yes HHG is intolerant of poor fitting parts and it is not a gap filler either.

Should have mentioned that we use quality single edge razor blades as a scraper. Very easy to control and you can really apply it where you need it too.

Don't worry, that shiny scraped ebony surface will adhere great.

I recommend flexing the scraper to hollow out the bridge by a hair. With a straight blade, the best you can hope for is perfectly flat, and any deviation will be convex and cost you some glue area around the edges. And since the soundboard is usually radiused in both directions, it needs a touch of hollow to mate perfectly. And a bit extra helps to counteract water expansion.

Roughing up surfaces is only for epoxy, which is primarily a mechanical bond. Especially important on metal, which has no micro structure for it to hook into. On my strain wave tuners, I use an x-acto knife to make deep score marks and work the epoxy into them with a toothpick before pushing the pieces together.

Experiments by the US Forest Products Lab back in the 40s found that fresh surfaces glue better than old ones, probably due to higher 'surface energy'. They also found that planed surfaces worked better than sanded ones. Scraped may not be quite as good as planed, IMO; much probably depends on how sharp the scraper is.

According to one article I read in the EAA 'Experimenter' magazine (aimed at airplane builders) even epoxies bond primarily through chemical attraction, and don't benefit from 'toothing' if it's not actually detrimental. That was the article that talked about 'surface energy'.

Thanks for all the input!

When my bridges fly off (they won’t) I’ll know the culprit is scoring them (it won’t be).

Hesh, who is your recommended single edge supplier? I use whatever is at the hardware store for doing gluboost repairs. Would be nice to have a trusted source.


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The only possible rationale I can think of for scoring the bottom of a bridge when using HHG, is that it may provide some place for the excess glue to go to and allow for a thinner glue joint. Hammer veneered veneers often have a toothed surface.

We just use our local Mom and Pop hardware store but beware there are some thin single edge blades on the market that suck. Dave had to look a bit lately for ones that are the traditional thicker ones since there are now thinner ones on the market too. I don't know the thickness I can't remember and I'm at home today if I remember tomorrow I'll check and report in here.

Beyond finish repairs too Brad a single edge razor blade can really refresh and clean up a very grungy fret board. I use them on the really bad ones, follow-up with OOOO steel wool and then hit it with Howards.