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Hi Everyone

Hope everyone is doing good. I’m getting to the end of my fourth build and was wondering what you folks do when filling gaps near the end of a build. I’ve attached some photos. I tried drop filling with ca glue but it slightly stained the redwood top. I had a light coat of shellac on it when I did the drop fill but the end grain still soaked it up. Luckily I sanded a bit of it away.

What would you folks do to fill the rosette on this redwood top?

All the best

Carlos

What finish are you planning on using?

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Break up some shellac flakes and place them over the gaps, and melt with a soldering iron or other heat source. Then scrape level, with tape on the scraper so only a portion of the blade is exposed.

Soldering iron 908S is great. 80W, temperature controlled down to 180C (and even remembers the temperature setting when turned off), comes with a variety of tips, and only $10. I have two, so I can solder wires and then melt hot glue onto them for strain relief without having to stop and change tips.

I use this trick with melting knives meant for shellac sticks, and a propane torch for the heat. It works whether you are using shellac sticks or just flake shellac. The melting knives make it easier to smooth out the shellac and keep the scraping to a minimum.

And it smells great!

Im not sure about the finish yet. I’m leaning towards an oil based finish(osmo) or nitro. Can I use a heating gun to melt the shellac? Does it just become a clear fill or would it match the top?

Thank you folks!

My concern about a heat gun would be where you point it. Do you plan to heat the flakes away from the guitar, and quickly move the molten shellac to where you want it, or put the flakes where you want them on the guitar and then heat them up? The heat from a heat gun can cause harm really fast; scorching the surrounding area, loosening other glue joints. This is why I heat the flakes on melting knives away from the guitar (using a propane torch) and apply the molten shellac with the knives. The approach of using a soldering iron right on the guitar has the virtue of keeping the heat only where you want it. I do use a heat gun on my guitars to heat up hide glue and tighten binding joints (as one example), but I am very careful to keep the gun moving to spread out the heat. It’s also really handy for taking apart a glue joint (same procedure and caution, but letting the heat linger longer so as to completely melt the glue in the joint).

Lovely instrument. A few ideas...

- When applying CA to redwood, spruce, mahogany, or other porose timbers (pretty much anything that is not a rosewood, ebony, or similar darker, denser timber), seal that wood with 2-3 light coats of super-blonde 1 to 2 lb cut dewaxed shellac (the Zinsser spray cans are 1 lb cut dewaxed super blonde shellac). As CA bonds nicely with shellac, and that shellac sealer with 2-3 coats applied does not permit even super-thin CA to penetrate to the wood (e.g., the BSI blue bottle stuff), a good bond will be made without staining or other excessive penetration of the superglue.

- A common practice is to mill the recess for the center ring, glue that slightly oversized ring in place with AR/PVA, fish, or hide, then mill the purfling channels. Seal the channels and 1/2-1 inch of the soundboard on both sides of the channels, insert the purfling and flood with super-thin CA, allowing an unaccelerated cure. Check if there are any holidays and spot-fill as needed with CA. Once fully cured, the top is taken from rosetting thickness (usually ~0.045 over desired final thickness for bracing) to bracing thickness (maximum desired thickness prior to voicing).

- If finishing with nitro, I suspect those gaps are small enough to fill with 100/0 (unthinned instrument lacquer) after an initial sealing coat of either super-blonde/'platina' shellac or vinyl sealer AND an initial drop-fill with 75/25 (add 25% thinner by volume) to ensure there are no trapped air bubbles down in the gaps.

- If contemplating an oil finish such as a brush-applied or wiping varnish such as Epiphanes clear high gloss, a clear shellac stick fill using a purpose-built shellac iron or CAREFUL soldering iron use (soldering station temp control or rheostat and experimentation to get proper tip temp) should be your go-to as explained earlier in the thread.

- I share Mr. Parker's concerns re: using a hot air gun. Applying hot air to a soundboard may dry things out enough to crack it, and wood can darken and discolor from even relatively low heat (<300 deg F) due to bound resins and sugars (it's why maple seems to 'burn' so readily even with fresh abrasives).

I generally use one of two methods depending on the width of the gaps. In either case the area around the repair is first sealed with shellac. Burn-in sticks work great if the gap is wide enough. They can be used with a heated palette knife. Someone gave me a beautiful Behlen burn-in kit that includes a soldering iron like tool that has a Teflon coated blade. This thing is excellent. If the gaps are too narrow to use the burn-in sticks I use epoxy mixed with furniture powders. After either of these the repair is scraped flush. If grain touch up is needed I'll use a graining marker.

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R.M. Mottola
LiutaioMottola.com

Author of the books Building the Steel String Acoustic Guitar, Practical Design of the Acoustic Guitar and Similar Instruments, and Mottola's Cyclopedic Dictionary of Lutherie Terms.

I don't check in here very often. If you need to contact me the best bet is via email through the contact page on my site.

The color of shellac remains unchanged when melted, but in narrow gaps like that it should be pretty much undetectable that there ever was a gap.

A heat gun would blow your carefully placed flakes away, so it would have to be used to heat something else and then touch that to the flakes to melt them. Or as Don says, heating with a flame also works.

Whatever hot metal thing you use, try not to touch the wood directly. Touch the flake and let it wick into the gap. And have more flakes ready incase it takes more than you expect, so you can get them in before the first one cools down.

And now that I think about it, you should fill the gap with dissolved shellac and let it dry first. In addition to pre-filling the smallest nooks and crannies, it should make the hot shellac more attracted to the surface in there compared to bare wood/glue. I always coat my surfaces with shellac, so I've been getting that benefit without realizing it.

Thank you all for the tips. I’ll try it on a scrap piece of redwood and see what works best for me. I’ll try some blonde shellac flakes with a soldering iron and see how that goes.

I also had some questions about finishing but I’ll start a new thread for that.

Best!

Carlos

One a side note, I like to apply two coats of a shellac wash coat to rosette and binding channels when using wood that has a tendency to weep CA glue into the end grain.

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West Systems has a Colloidal Silica used as an epoxy bulking agent that fills gaps when mixed with epoxy. I've used it on a number of drop filling tasks.
Epoxy, nitro or EM6000 lacquer slightly thickened with this will fill gaps invisibly, typically with one drop fill. It's been my go-to drop filler for binding, rosettes or the random deep grain line before and during topcoat finishing. I seal bare wood with dewaxed shellac first, or apply directly over top coat. While typing this I'm thinking the silica might even work well in the shellac itself, but I haven't tested it.

The product description say it dries an off white, but I've found it to be invisible as a tiny gap filler. West Marine and amazon have the best price.
https://www.amazon.com/Boating-Accessories-Colloidal-Silica-System/dp/B0000AY1OC/ref=asc_df_B0000AY1OC?

Use a respirator when working with it- it explodes out of the can when you open the lid and you don't want to breath it.

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Measure Twice,

Karl Borum

Dissolved shellac as a filler takes a long time to finish shrinking back. It's hard for the alcohol to get out of anything more than a very thin layer. That's why they came up with melting shellac sticks.