I have one bit of shift surround wood I'd like to refinish because it's cracked. It wasn't clear to me, however, whether the cracks are in the wood or in the very thick 'lacquer' coating (clearly not actual lacquer) on the wood. So, in the spirit of exploration, I had acquired a cracked shift knob to look at how difficult it would be to 1. Take off the coating, 2. Refinish the wood, 3. Reapply the coating (or some coating).

So, I have some information on 1. Rooting around, various places suggested that the coating is a UV cured polyester. Previous experience with both polyester and epoxy finishes suggests that only the flaming heat of the apocalypse will remove them, but I put bits of flaked coating into denatured ethanol, lacquer thinner, acetone, and naptha to test the information given. Waited a week and...


Bits of coatings placed in chemicals.

... nothing. The coating sneers at ethanol, lacquer thinner, acetone and naptha.

So, on to flame, or the heat gun equivalent. Bottom line, the coating will soften the coating sufficiently so that it can be removed (a bit laboriously), as reported by others. I don't particularly know the temperature at which the coating softens. Needed temperature appears to be hotter than a hair drier, and my heat gun risks marking the wood with enthusiastic application, though the explosive fracture of the coating under high heat is entertaining and doesn't appear to damage the wood or veneer. I plan on trying the oven when I have some time. But, even with the most enthusiastic application of heat, the veneer did not debond from the plywood substrate on the sift surround. Good news.


Shift knob stripped with heat.

It became apparent that the cracks in my test articles (cracked shift knob and cracked test trim wood) are not in the wood and are solely in the coating, more good news. Even if that had not been the case, I suspect the burly nature of the wood would hide this with restaining/refinishing.

So, bottom line, common solvents don't work, heat does work, the cracks are probably in the coating and the water intrusion will either sand out or will hopefully look like burl after the refinishing.

Next stop, determining the approximate temperature of softening to allow potentially faster removal, sanding and refinishing.

 

I read somewhere, don't ask my where now, but someone suggested using superglue. It was a suggestion from someone who repaired guitar's for a living. They suggested the crack is probably in the coating as you have no doubt discovered.

 

I read the post then saw Location: Durham and immediately thought - here's a man with waay too much time on his hands. Maybe he could come and detail my XK. Sadly it's the wrong Durham.

Sorry, Dale. Couldn't resist it.


Interesting that you found the damage was only in the coating. I've seen a lot of cracked shift surrounds and always assumed it was the veneer that had cracked.

Graham

 

Dale, I love it. Thank you for sharing this! Would you be open to testing solubility with commercially available strippers that advertise good for poly? I'd love to refinish some of mine, sure wont have the guts/patience to use heat, and figure we can find a far easier way than sanding.

I was about to offer to look downstairs for a stripper I liked, but rembered it was for lacquer.

Mike, I too have heard that trick. You know what? I live next door to a luthiere. Doubt many stringed instruments are poly coated, but asking is free. Standby on that one.

John

 

Dale, just did some research. Lots of places say: equal parts denatured alcohol + lacquer thinner. Maybe there's some kind of wierd synergy? Could you test a mixture?

Thanks,

John

 

Absolutely, back in a week.

 

Sanding doesn't seem to be a practical option in any case. The coating is way too thick, and the veneer is way too thin for my level of patience with this. My hope with the heat is that at some oven like temperature, it all gets soft is easily peeled off. The limit to the heat gun is the local application produces local flakes.

And I think there are many guitars with a similar coating. My understanding is that they use heat.

 

Wrong Durham? My Durham has minor league baseball and a major motion picture. Yours?

On the detailing, I'm game. I'll include the expensive flights for free at my usual consulting rate. Bonus, I'm middle aged and slow...

 

Superglue is an interesting option here. Will try this this weekend with a well bonded bit.

 

Dale and all,

Visited the luthier, great guy. To my surprise, he endorsed crazy glue for crack repairs, lacquer or polyurethane. He invited me in to view a recently finished job, an Ovation guitar that had a crack the surface.

I kind of got the impression that superglue was a more cost effective solution, but don't get me wrong - it looked nice, certainly nice enough for a car.

I asked him about stripping polyurethane. He responded that lacquer thinner or any commercially available stripper would work fine.

This is curious. He is an expert to say the least. When I brought this up,his 1st question was: "polyurethane or polyester". I certainly got the impression that the later is a much harder surface to work with. I didn't pursue that line of questioning.

Polyester, polystyrene, polyethaline, I'm starting to question my memory of the last category. I'll find out and update . Either way I'm beginning to wonder if we've maybe mislabeled the finish (and I was the guy who called the Jaguar wood refinished and posted what they said - polyurethane).

**edit: it was polyester resin, common on cheaper 1970s electric guitars.

We will figure it out!

John

 

Darn, looking more and more like polyester resin given Dale's test, manufacturing requirements, durability requirements, plus the fact that these finishes always seem to crack not scratch (aka: polyester vs. Polyurethane performance results).

I'll check with luthier again, but I can only find heat application as the only way to remove polyester resin.

John

 

Dale,

Thank you for your great post and inquisitive mind! I respect your scientific approach!

Below is some information I have posted before in other threads on veneer repair, but before I copy and paste that info, please allow me to mention that in my experience panels with the veneer glued to a plywood substrate tend to be very rugged and the veneer bond to the plywood is very durable and can typically survive commercial strippers (methylene chloride), which will often break down even UV finishes but may require repeated applications.

My one point of caution is that veneer panels with a metal substrate tend to be much more fraglie; the glue bond between the veneer and metal backing is not durable and attempts to use stripper or heat can cause the veneer to separate from the metal and crack into small pieces. Don't ask how I know....

OK, so on to the previously posted info that may be relevant:

"Lacquer" has traditionally referred to nitrocellulose lacquer, which was used on acoustic instruments and in Jaguar interiors for many years, probably through the '70s. But over the last few decades, automakers and the larger guitar makers have switched to high-solids Ultraviolet light (UV) curable finishes to speed up production time and also reduce the emissions of Volatile Organic Compounds to comply with increasingly stricter EPA regulations. These finishes cure in just minutes under UV light, and can be sanded out and polished just like traditional lacquer.

It is possible that the finish used on the wood in X100s was solvent-based, but it is not nitrocellulose lacquer. It is probably a high-solids urethane or polyester. The finishes used in more recent Jaguars are almost certainly water-reduced UV curable urethanes or polyesters, since these have become the industry standards. Even some smaller guitarmakers such as my favorite, McPherson Guitars, are now using UV finishes. Here's an example of another small luthier:


Hardwood floor refinishers are now using this type of finish:


These modern finishes can still be repaired by a good luthier or crafty do-it-yourselfer by the superglue (cyanoacrylate) "drop fill" method. Depending on how much the cracked areas have lightened, a little stain or aniline dye may need to be worked into the cracks first. This method can even help when the wood itself is cracked. If your cracks are tight, luthier suppliers like Stewart-McDonald (stewmac.com) offer low-viscosity cyanoacrylate that will flow into very tight spaces. Here's a video of master luthier Dan Erlewine showing how superglue is used to repair a finish chip:


Cheers,

Don

 

Fantastic, Don. Appreciate you bringing this to this thread and the link. The superglue technique is quite interesting, especially since we have variegated burl, not some organized pattern which might require more skill in the back stain or back fill.

An experiment this evening with uniform heat was a disaster. Local application works fine (heat gun over coating). Global application of heat in the oven starts singeing the wood at temperatures lower than those necessary to produce broad softening (~ 200 C - 230 C).

Wife is long-suffering.

 

Overdone shift knob pictured below. Oven temps of ~180 C (~350 F) are not enough to soften the coating. At just over 200 C (400F), we get the lovely smell of singed wood and baking shift knob. Since that is not a particular consequence of the heat gun approach unless wielded immodestly, the overall heating approach in the oven doesn't seem to be viable.

Also not viable is lacquer thinner+alcohol. Been a couple of days, but see no action on the chip. I thought about trying MEK and methylene chloride, but tomorrow (Nov 24th) a ban goes into effect in the US on use of methylene chloride in paint strippers. Coincidence, but not particularly convenient for this investigation..

On superglue, the coefficient of thermal expansion is about 0.00023 m/m/C while the value for polyester is 0.00012 m/m/C, a factor of 1.8 lower. So, any superglue that gets under the polyester may eventually lift it up and will make the local separation between the two parts of the crack worse. I was hoping that superglue expanded less than polyester under temperature because local cracking happened in the polyester itself, probably from thermal expansion/contraction. Looking at the guitar repairs around, I'm sure it works to fix the crack, but I'm not sure how long it would work in a car in NC with 55 C temperature swings between winter and summer.

So, I went looking for a thin UV cured polyester to fill the cracks which would have the same thermal properties as the rest of the coating. Turns out that the people who use surfboards also have this problem, and there is a uv cured product for this

https://www.solarez.com/product/fly-...thin-bone-dry/

So, ordered the UV cured polyester and will be trying it. This seems like a fun and useful material anyway.


One shift knob, lightly toasted

 

Thank you for all the effort put into this investigation. Like many of us, I do have an unsightly crack in the dash veneer coating. Luckily it does not go into the wood itself.

I was just about to try the super glue (gel type) when I read the last post by crbass. Thank you for covering all the bases. I’ll be trying the surfboard fix myself.

But what to do with all the extra polymer fix-it ? I guess I’ll go into the surfboard repair business. Landlocked Oklahoma is sure to be a surfers mecca.



Z

PS. crbass, what are you using to do the UV curing ?

 

Already have UV lights, several intensities. One type requires eye protection. Will try the cheap, find the AC leak one first.

 

Hi Dale,

You may be overthinking this, since the coefficient of thermal expansion of wood is 0.000003 m/m/C parallel to the grain and 0.000030 m/m/C perpendicular to the grain, a factor of 10. So the cracking is probably caused by the wood expanding and contracting to a greater degree than the finish.

However, I am excited about your surfboard discovery and can't wait to see how the repair goes!

Cheers,

Don

 

Perhaps. It's happened before...

But the wood base on the shift surround appears to be plywood, more isotropic, depends on the basic construction and species (but is ~0.000006 ish m/m/C) and the aluminum is isotropic (around 0.000024 m/m/C depending on alloy). I doubt the burl does anything other than go along for the ride, and the grain is highly anisotropic anyway. So, it's probably the other way around, the finish is cracking because it is less thermally stable than the substrates.

 

Another potentially significant variable is humidity. Wood expands and contracts differently with and across the grain depending on moisture content, and burl changes dimensions internally due to the grain running in many different directions. If you've ever seen a piece of burl veneer that has been sitting without being in a flattening press you can see how its internal stresses cause cups and bumps to form in humid conditions, and splits to form in dry conditions. These effects should be greatly reduced under a polyester finish, but since the plywood substrate is unfinished, it can absorb moisture from the atmosphere. Plywood is more dimensionally stable than solid wood since the layers constrain one another in length and width, but plywood is unconstrained to swell in thickness in humid conditions. Just helping you overthink!

Cheers,

Don

 

Appreciate that, the more overthinkers the merrier. Or something like that. Usually need all the help I can get.