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This is my third nitrocellulose lacquer job so I’m not an expert. I am pretty good at having gaps in rosette and putting nicks in white tops though. My problem has always been leveling after the drop fills with lacquer or with Glu Boost CA. Sand through was always a problem. I wanted to share what I’ve learned this morning. The most common problem I’ve encountered in fixing them is, they look smooth, but when build coast are added, there are still bumps. Three methods I used this morning has taken away all the anxiety (most anyway) in doing drop fills.

1. My luthier buddy Joe Mendel of Mendel Stringed Instruments in St. Louis recommended a small wooden sanding block. I made one this morning out of scrap tone wood. It’s small, easy to grip, easy to control and easy to bias. The results were amazing when sanding down drop fill bumps. I attached P400 pressure sensitive to the bottom:



2. The second thing I’ve learned is to drop one or maybe two lacquer fills, then level sand BEFORE adding more lacquer. This prevents the 10 drop lacquer hump and shows exactly where the next drop needs to be. I had been doing 8-10 brush on session before sanding, creating more work, and bigger humps.

3. Lastly, I modified the scotch tape razor blade method. In addition to the scotch razor blade, you place a strip of scotch tape on either side of the hump, (works great on a long gap along binding too) the material removal is hyper focused on the hump and won’t remove ANY of the surrounding material. After scraping with razor blade and tape, switch to sanding block until you start sanding into the tape (P400 grit). Remove the tape strips from the wood and finish sand with the small block. A dozen light swipes is all it takes.

This morning’s rework session (reworking the humps I lacquered over) was very satisfying and results were excellent. Examining the surface with a light at a low angle helps immensely. The sanding block was the single most excellent tool for leveling the small drops/ humps and took all of 10 minutes to make. It went so well, I ended up dropping and leveling about 30 small defects in three hours. That’s in addition to the 15 crappy humps that I reworked. Hopes this helps some one as much as it helped me!

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Measure Twice,

Karl Borum

Good discoveries!

I've been using the Kovax Tolecut system I got from StewMac - at $25 it's kind of pricey but I only use it for spot repairs and after 3 or 4 years I'm still on the original set.

I use masking tape (233+) on my razor blades which holds them up higher than scotch tape. I have to flex the blade to get it down to the level finish which makes it easy to be precise. Perhaps the extra height is why your extra layer of scotch tape is working for you? Doesn't matter how you do it as long as it works

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Steve Smith
"Music is what feelings sound like"

The extra layer of tape helps, but the sanding block is dope!

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Measure Twice,

Karl Borum

Good going Karl and see how much you are learning too!

We scrape with safed razors and then sand using hard rubber blocks made from horse stall mats. This material has a bit of flex but very little.

A proper buffer is a huge help here too as the last step.

We've never had the need to try glue boost and instead use quality Bob Smith CA often found in hobby and hardware stores under private label. We of course also avoid any accelerator use when wishing the CA to stay clear.

I admit this is what the boys used to describe as an 'inside baseball' discussion, but here goes:

The GluBoost (aka, GB) materials are a designed system optimized for synthetic finish repair. That is their claim and we have found it to be an accurate statement. The reason why we pay the very significant premium for the accelerator, various fill and finish CAs and powdered CA colorants is because they largely do away with the bad behaviors of even the best of the commercial CAs like the BSI and Mercury product lines for a very specific set of use cases of interest to those doing guitar finish touch-ups.

We keep three GluBoost products on every bench (accelerator, thin Fill and Finish, medium Fill and Finish), along with BSI Super Thin, Medium, Extra Thick Max Cure, Super Gold, and their CA remover.

Why the extra, seemingly redundant products? Let me explain.

- GB Thin Fill and Finish may be padded on with Viva paper towel pads (really, a full sheet cut into 16 segments) to build full depth replacement finish... accelerated after each set of cross-coats, and with a predictable shrink-back at cure, scrape can occur within 30 minutes (matching softer polyurethane finishes for hardness) to three hours (well-cured 2K or 3K urethanes or polyesters). This saves calendar time on repairs over what the shop historically (aka, BT versus AT) saw with BSI or mercury unaccelerated cure. Further, neither of the GB Fill and Finish repair-optimized CAs will see a color change with or without GB acceleration... at least over a better than six year timeframe.

- BSI Super Thin and Medium show a markedly different and thus variable final cure hardness over GB Thin and Medium Fill and Finish. This is a show-stopper for finish work, as it means we have to guess as to where the BSI product is in bench life (life after opening), and then scrape and sand at the point where hardness of the repair matches hardness of the finish. The bench lifecycle for BSI products versus GB is as alkaline versus ni-cad battery life... BSI products age by sucking up moisture from the atmosphere, resulting in a gradual change to longer and longer times to reach peak hardness, while GB Fill and Finish products have very consistent cure behavior and hardness profile (the difference between initial cure hardness and final cure hardness when sanding) until - suddenly - they die in the bottle.

I could go on, but for those that have some desire to simplify shop logistics, CAs and support systems represent something of a quandary for minimalists. In terms of roles in the Germantown shop:

- GB Thin Fill and Finish: drop fills and built large-scale replacement finishes on synthetic finishes; fill over shell channels; overcoating lacquer or shellac toner coats to seal them against additional CA top coats (you will still see some color pick-up over BSI Thin or Medium, but not nearly as much).
- GB Fill and Finish (essentially a medium CA): finish repairs of over 0.008" such as building out a filler coat such that it may be colored and top-coated
- BSI Super-Thin: quick glue jobs that do not involve drying finishes and situations where penetration is the overiding factor in use (such as reattaching polyester finish which has separated from the filler coat or underlying sealer coat on something like a modern Fender)
- BSI Medium: glue jobs requiring greater open time, but where viscosity is not a factor
- BSI Extra Thick: K&K and JJB transducer mounting (BSI Gel has a bench life of weeks with no real benefit over the Extra Thick which will last a year or more in bench use)
- Viva paper towels: much smoother than Bounty or other premium brand, somewhat less absorbent paper towel seemingly optimized for use in padding on CA

On sanding... we never sand until the finish is fully leveled with the scraper... which means we carefully take the repair down until the boundary between existing finish and the repair is just removed/flush. Leaving even a 0.0005" mound will show signature on a buffed finish, with a significant hardness mismatch making it more evident.

Mr. Breakstone's suggestion to use stall padding as sanding block is a good one, and any Tractor Supply or other farm and feed store should have recycled, vulcanized rubber mats available. Another alternative is old-school automotive gasket material made from ground cork and oil/solvent resistant rubber glued to a piece of marine plywood sealed with epoxy.... both materials hold up well in water as well as mineral spirits (the lubricant of choice for rubbing out well-cured phenolic/tung oil varnish with rottenstone and other, finer abrasives).

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For the times they are a changin'

- Bob Dylan

If I used a lot of CA, or did a lot of finish repairs I might be inclined to try some of the premium offerings, but instead use Loctite super glue. I buy the thin stuff and as it absorbs moisture it becomes medium and then thick CA, and during this metamorphosis I have in the mean time opened a new bottle of thin. I'm sure there are better products, but as long as it doesn't give me the Chinese yellow the really cheap stuff turns I am O.K. with it.
If I want a quicker cure time I will give the joint a dusting of baking soda.

For a "hard block" a piece of P-lam glued to a block of wood makes a good surface to stick PSA papers to.

A technique that comes in handy at times when levelling fills is: use an appropriately-sized piece of plexiglass as a sanding block.

Instead of covering one whole face with sandpaper, adhere just one very small square of sandpaper in the center of the face, about the size of the high spot you’re sanding or a little smaller.

Now as you sand you can see exactly what is being cut and it’s easy to ensure you don’t sand anywhere disadvantageous. You can stick a strip of tape or two the sanding face as a depth stop.

I think I saw Ian Davlin doing this in a Stewmac video.

RE gluboost, I also find it’s no better than regular CA for drop fills. But I have found it useful for its long open time, which allows it to be almost painted or wiped onto a broad surface, cured with the matching accelerator, another layer painted on etc. You can rebuild an entire finish on a peghead, neck heel etc very quickly using this method. It can be a viable way to get a neat-looking finish on a job that doesn’t justify the cost of actual refinishing.

My experience as well, exactly. I’m almost happy to see I'm not the only one that had Fill'n Finish bottles suddenly die on me. It then refuses to cure as fast as previously, so first time it happened I believed I did something wrong (I was using black powder to rebuild cracked and detached poly) and kept on trying until I just replaced it with fresh stuff. It all went well with the fresh stuff, so… lesson learned. This is very useful stuff. I quit using regular CA after having tried this.

As a general rule I just throw the bottles away after one year and order fresh bottles as the need arises.


Pierre
Guitares Torvisse

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Pierre Castonguay

My non-professional reaction to OP's question: I use nitro to finish and any CA I use as drop/gap filler gets scraped with Stanley knife blades. I have both single-edged razors and Stanley blades on my bench and I've found the less-flexible Stanley blades superior at leveling CA buildup.

But I speak only for myself and my limited experience.

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Peter Havriluk

Some of this could go in the tips and tricks thread. I like to use a razor blade with the sharp edged rounded off and two pieces of Scotch tape placed, or wrapped, around the sharp edge of the blade separated about as wide as the drop fill. The tape acts as an edge protector so that you can easily and quickly scrape down the bumped up drop fill till at least the thickness of tape, which is pretty thin. Then I carefully take that further by taking another razor blade without the tape and bending it into a curve like you would a scraper blade. I then finish off with sand paper wrapped around an artists eraser.

I really like GluBoost too. I started using it a few years ago now and there is no going back for me. I've never seen the excelerant distort the clearness of a GB fill.

I would add using a mgnifier hood or clip-ons if you wear glasses. If it looks good through a magnifier, it will look GREAT to the naked eye.

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formerly known around here as burbank
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http://www.patfosterguitars.com

Absolutely. Didn’t think to mention that since its second nature for me to grab my Optivisor any time I’m doing detail work.


Steve

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Steve Smith
"Music is what feelings sound like"

Added note: (i do this as well), round the blade corners to avoid nicks.