Aluminum hydroxide? Scotchlite? Other crack fillers?

[Nick Lewis]

I'm going through the Iris Carr crack repair/retouch course, and I can't recommend it enough. Absolutely inspiring to see her methods and the results she gets. 

When there are minor pieces of missing wood (fibers or thin slivers), she mixes Scotchlite glass beads with some color and hide glue to match the color of the wood, mixes it into a paste, and fills a little bit shallow, then proceeds with thickened deft and finally retouch colors. This is different from hairline cracks, which would just use thickened filler.

I've been experimenting with a few things, and wonder what experience others have. I've tried mixing color and hide glue with scotchlite. It seems to always be really opaque, and I'm still working to get the color right. Other discussions point to poor scraping and I'm not always good about getting it below the surface. So I've been trying aluminum hydroxide, a similar idea of using the powder with color and hide glue. It seems to work OK, but is slightly more translucent than the glass bubbles. I'm nowhere near good enough to make things "disappear" so at this point I don't have enough experience to know which one gives better results, just that they're different. I've also tried mixing with my thickened filler instead of hide glue, still to a consistency of paste, but I feel like the hide glue would be more reversible to just wash out with hot water. I can extend the working time of glue by working under an IR lamp as I mix the colors to try and match the wood. 

There are lots of variations on this theme. Do you color to try and match the wood? Do you just use thickened varnish? To me that always leaves a dark void that stands out. I know there are lots of other posts on related topics, and I've been reading through them, but since this is always an evolving field I thought I'd bring it up again and see what people are doing. 

[Jeffrey Holmes]

Iris is very talented and a very good teacher.  

Whatever system, for modification of a system, of fill one ends up working best for any individual may be a bit "personal". I think the result and stability is what counts.

My own toolbox:

I occasionally use Scotchlite and find it works well when building up voids too deep for filler varnish alone. I color it to match the wood itself (not too dark or intense/opaque) before using filler vanish to level the area that is being retouched. I honestly don't find it's needed all that often, but when it IS needed I haven't found a better system.

When I feel it's required I use aluminum hydroxide in filler varnish to give the varnish a bit more "body".  I do not tend to add color to it (colored touchup is added over the fill once it's leveled).  It has the advantage of modifying the fill so that scraping it level is easier to do (especially if it's caught at just the right time) and as long as not too much is added to the filler varnish it is not at all noticeable when dry.

I personally don't like deft because (although it's easy to level) I find it tends to continue shrinking longer than I'd like, but I know several fine restorers, including Iris, who get along with the stuff very well.

My 2 cents for what it's worth.   

 

[The Violin Beautiful]

I haven’t tried Scotchlite but have been hearing good things about it for years. A colleague gave me a jar of aluminum hydroxide at the Oberlin restoration workshop a long time ago and I’ve used that ever since for filling. I don’t add color to it, either, only applying color to final layers of touchup varnish. I used Deft a bit in the past but never felt satisfied with it because of shrinkage. Thickened shellac (left uncapped in the workshop for a day or two) with aluminum hydroxide has become my standard filler varnish. My touchup varnish is a little harder, so I’m thinking I may make up a mix with aluminum hydroxide for comparison when my current jar runs out. 

[Nick Lewis]

23 hours ago, Jeffrey Holmes said:

as long as not too much is added to the filler varnish it is not at all noticeable when dry.

 

Ah, reading other posts, I think that’s what I’ve been missing. I’ve been adding significant amounts such that it begins to get milky or even opaque.

I haven't tried deft yet, but in my experience I’m hard-headed and have to convince myself that something doesn’t work well (or does work well) before I give up on an idea. 

Before I do that, though, I need to play with matting agents to start getting my sheen right. 

[Nick Lewis]

21 hours ago, The Violin Beautiful said:

Thickened shellac…with aluminum hydroxide

I’ll give that a try! I think I misread earlier posts about Al(OH)3. I was adding significant amounts to 1# cut retouch varnish. Adding to my thickened retouch makes much more sense. Thanks! 

[Nick Lewis]

So is the idea with Al(OH)3 that the thickened filler won’t sink as much as it dries and necessitate extra layers? Or does it also help the scraper properties for getting surface texture right? 

I’m sure I’ll learn with experience, but I always like to figure out why I do what I do. 

[Jeffrey Holmes]

17 hours ago, Nick Lewis said:

So is the idea with Al(OH)3 that the thickened filler won’t sink as much as it dries and necessitate extra layers? Or does it also help the scraper properties for getting surface texture right? 

I’m sure I’ll learn with experience, but I always like to figure out why I do what I do. 

Both, when you get the mix "right".

I personally don't use shellack alone for fill but it is certainly a component of my filler varnish.

[Nick Allen]

Anyone here ever notice that the Scotchlite filler causes massive reflection issues? 

Perhaps I'm not skilled at using it? But anytime I have a void to fill and use it, along with coloring it beforehand, afterwards it just looks terrible when you look at the plate from a different angle. 

What is the main feature/benefit of Scotchlite?

[Nick Lewis]

38 minutes ago, Nick Allen said:

Anyone here ever notice that the Scotchlite filler causes massive reflection issues? 

Interesting, this is actually the reason that Iris Carr stated that she used scotchlite; the reflectivity was better matched to wood than other alternatives. I’m still really new in my use and overall experience so I can’t really say. She did only use it on tiny gaps, such as missing fibers, so maybe it’s good up to a point but if the area getting filled is too big then it stands out? 

I had been curious about this too, so looked more into what exactly these glass bubbles are. It sounds like they’re actual hollow spheres (which makes me fascinated how they manufacture these in the first place), so naturally you’re going to have some interesting refractive stuff going on, transitioning from a curved glass surface to a hollow core and back out, so passing through different indices of refraction and curved surfaces makes it interesting. If nothing else, they’re very opaque. 

I wonder if what you use to color it makes a difference? I’m still experimenting with different things for that. 

[Jeffrey Holmes]

18 minutes ago, Nick Lewis said:

I wonder if what you use to color it makes a difference? 

Yup...

[Shunyata]

Scotchlite is what they use to make reflective road paint.  So the “different angle” phenomenon makes perfect sense.

It is made up up glass microspheres.  Seems like a BIG health risk.

[Jeffrey Holmes]

5 hours ago, Shunyata said:

Scotchlite is what they use to make reflective road paint.  So the “different angle” phenomenon makes perfect sense.

It is made up up glass microspheres.  Seems like a BIG health risk.

There is certainly a learning curve, but the reflectivity issue can certainly be overcome or minimized. The same danger is present when simply retouching if you don't get the opacity/clarity just "right" or your glue line isn't clean and tight.

Yes, like several other materials and solvents we tend to use, care is advised when using the stuff (like a mask, gloves (PPE) and minimizing airborne particles).  

[The Violin Beautiful]

On 5/24/2026 at 12:19 PM, Jeffrey Holmes said:

I personally don't use shellack alone for fill but it is certainly a component of my filler varnish.

I agree that a mixture of resins is superior. The only reason my current batch is just shellac is that I was running low on retouch varnish but had plenty of Bullseye that wasn’t being used, and I had a lot of filling to do at the time. I get years out of each batch I make, so I tend to put off making the next one until I absolutely need more!

[Jeffrey Holmes]

4 hours ago, The Violin Beautiful said:

IThe only reason my current batch is just shellac is that I was running low on retouch varnish but had plenty of Bullseye that wasn’t being used, and I had a lot of filling to do at the time. I get years out of each batch I make, so I tend to put off making the next one until I absolutely need more!

Yup... can relate!

[Advocatus Diaboli]

On 5/25/2026 at 8:34 AM, Nick Allen said:

Anyone here ever notice that the Scotchlite filler causes massive reflection issues? 

Perhaps I'm not skilled at using it? But anytime I have a void to fill and use it, along with coloring it beforehand, afterwards it just looks terrible when you look at the plate from a different angle. 

What is the main feature/benefit of Scotchlite?

Blanc fix is your friend there.  I’ll often use scotchlite almost to the surface of the wood, filler varnish to get the rest of the way up to the surface, watercolor in some wood details, and then fill and retouch on top of that.  Gives a bit more movement than scotchlite alone.  At the end of the day I want my work to not stand out, but pretending damage never happened isn’t what I’m after. 

[Marty Kasprzyk]

Wood fine sawdust work as a filler?

[The Violin Beautiful]

6 hours ago, Marty Kasprzyk said:

Wood fine sawdust work as a filler?

It works in some settings, but it tends to be darker than the uncut wood once glue is added. If the varnish is opaque or dark enough it can work, but with a light or transparent varnish, it can end up sticking out more. 

[FiddleDoug]

I don't see any discussion of lycopodium as a filler here. That was one of the old standbys. Anyone with experience using Scotchlite, aluminum oxide, and lycopodium care to compare all three?