They are probably a result of some initial contaminant which will reject the initial lacquer and generally defy any attempts to drop fill of spray over. You can get a coat or a drop fill to cover over the hole but once you start buffing out with heat these mini fish eyes return to ruin your day.
There are a few ways to get the lacquer to take short of taking the whole thing back to a where the crap that is causing the problem is: I have used a sharp (syringe needle) and a magnifying goggle set to dig into the problem area down into the layers and open it up for a mini drop of thin CA into the hole to seal off the contaminant which allows subsequent appropriately tinted nitro to "take" to the CA and surrounding nitro.
Otherwize you may try a lacquer drop fill with a goodly proportion of Retarder in the lacquer to help it bite and flow.
I also just noticed you let the thinners evaporate from your nitro before drop filling. This will not assist the lacquer to melt in and bond as well and is contrary to our practice to drop fill with many thinned coats rather than a few thick blobs which sink in when buffing heat occurs.
Plenty of way to skin this apple.
What kind of glue was used for the inlay? Looks like it could be a couple of tiny air bubbles. That can easily happen at the edges of inlay if CA is used to glue them in.
Peter, the reason you were doing the drop fills was that you had an imperfection in the top coat probably cause by a contaminant under the original finish. Otherwize you wouldn't have had the problem in the first case. The drop fill will not take to an exposed pre-existing contaminant so you just get more of the same when you try and fill in the craters.
If the overlay was finished with an aluminium oxide abrasive and some of this got fused into the surface of the overlay when they were sanding it flat this may account for the source of the lacquer not adhering to this area.
That's what is looks like to me as a professional nitro sprayer of some 15 years. However, if I just don't get this can someone else take carriage of it.
It appears as if you have a couple options at this point:
1. Live with it [it's a micro blemish which can be successfully addresses with a touch from the tip of a Steinway re-finisher's pen]
2. Take the finish off down to the substrate and try again.
There exists the possibility that the source of the overlay, or someone afterwards, didn't do a good enough job preparing the substrate for final finishing. But that's water under the bridge. The debate of 'how & why" could continue for days, but it won't correct the perceived issue.
Rusty is MY defacto go to guy for finishing advice. His advice and assistance are worth their weight in gold. His experience and knowledge are unparalleled when it comes to finishing & refinishing.
It appears as if you've done an otherwise exceptional job of finishing the headstock. Those micro blemishes would not stop me from considering a guitar with similar 'niggles' for purchase.
ALL guitars, no matter how careful we are with them, will have dings & scratches within a few months anyway. Learning to accept & live with them is part of guitar ownership. Absolute perfection is not a healthy expectation.
In closing, "do overs" are a common event in the craft. There's no shame in it whatsoever.
Best of luck :)
Quote: "Any idea why these appeared?"
There are a number of possible causes.
These range from lack of Routine Draining Maintenance at the Compressor, inefficient water trapping of moisture in the Compressed Air Supply Chain or Line, tiny drop[lets that can be picked up in the pipes resulting in minute holes of moisture contaminating the material, and emerging from within it, during or after application.
Insufficient Drying of Repeatedly Applied Layers of Lacquer resulting in built up un-evaporated solvents beneath, gradually emerging through the latterly applied Layers (the solvents of which re-excite the underlying un-evaporated solvent) in the form of minute bubbles, as they try to escape through an ever thickening surface.
Or indeed Insufficient Atomisation of Material during Application, resulting in an unsatisfactory breakup due to an un-optimised Air Supply resulting in tiny pockets of air being introduced, between thicker material as its applied, with a slower than ideal speed of lay down.
Sometimes these faults can be immediately detected on the surface of the Product, sometimes they can be so tiny and showing up less obviously, depending upon the underlying background hue, that unless an experienced eye is involved, they are easily missed. Sometimes they are present but undetectable, however when the surface on the New Material is Polished, it opens the top of any existing bubble and creates a pin hole within the material, and thus suddenly becomes visible, and suddenly seem to emerge from the Lacquer.
I note you noticed them after your Polishing.
Rusty has already given you his commendable expert advice, unless you want to rub everything right down and start over as Paul suggested.
There is one thing to bear in mind here. Rusty has acquired over a very long time, a wide ranging skill set and experience of dealing with all kinds of small, localised, vagrant Finish Issues.
He not only has a personal arsenal of different techniques he can employ, he just as importantly, is highly sensitive to the Ideal Viscosities that are necessary for the Flow of New Material to Properly Enjoin the Existing Finish.
It all seems very easy, and it is, if you have the Depth of Experience. If you don't posses these skills which take time to acquire, and thus find yourself frustrated as a result. Only you have the nous to determine your Level of Confidence, and Paul has given the best route to follow, should that Confidence be Insufficient.
To make you feel better.
If you have been, and are frustrated by these Finish Problems.
Although for various reasons I am unable write about or refer to my own "business interests" directly.
"Just Imagine" in your mind for a moment the problems, I "just might" have been embroiled in and with Yesterday, Last Night and Today.
Imagine a Similar Finish Problem to, but not actually Pin Holing, but just as much a Problem in its way. Imagine you have Flushed a Huge Holding Vat with Cleaners, and send a "Pig" throughout the entire supply system of Stainless Steel Pipes, to Scour Clean the Inside Bore.
The Holding Vat has Two Pumps, one for Supplying a Continuous Flow of Material to the Point of Atomisation, the other to Switch to in times of Breakdown and to enable Routine Pump Maintenance without interrupting use of the System. All the Pumping Systems and Air Supply Side check out as O.K. and confirmed by Lab Tests.
You have subsequently filled from a Massive Transportation Vat, the Huge Holding and Pumping Vat with New Material from a Major Supplier, brought it on line and started to Apply the Finish to the Product. You have a Massive Curing Oven, Half a Mile long and with Hours of Product Within it. After the Product finally emerges from the Oven, and goes through Final Polishing and Inspection the Finish Problem is Highlighted.
It looks to you like the problem is actually in the Material itself. You get a Lab Analysis to Confirm your beliefs double quick time. You need to Dump the Faulty Batch of Finish which is the equivalent of many, many Oil Drums of Material and Re- clean the System. A Huge Cost Financially. You get a Brand New Batch delivered by Vat of Material Supplied double quick time.
Meanwhile. You have a Continuous Supply of Product awaiting Finishing. So you Flush a Another Completely New Alternative Supply System, (you have many at your Disposal all the Time) with another Huge Holding and Pumping Vat with Cleaners, and send a "Pig " around the system to scour the Bore of the Stainless Steel Pipes. You load the New System with the Second New Batch of Material and Apply the Finish.
In the back of your mind, you're wondering whether the hundreds of thousands of pounds of Lost Material and Lost Production is yet over the One Million Pounds Lost point, and figuring the Ratio of Re-Finish Successfully Repairable Products that can be Reworked and Redeemed, to the Ratio of Scrapped Product, and trying to Decide Whether you would be Best Off in the Long Run to Simply Scrap the Entire Production Altogether.
At this point, the New Products emerging from the Curing Ovens, Finished with the Second Brand New Batch of Material Emerge. And the New Material, Applied from the New, Alternative Supply System, through the Alternative Pipework, creates the Exact Same Finish Fault Issue in the Product. Where do you go from here? It's all very Dramatic, but believe me, these Problems are Resolved very Quickly Indeed as they are so costly, it's like Monopoly Money down the Drain.
But if you are tempted to feel frustrated at your problems, just imagine what it's like for some people.
And hopefully you will be spurred on to Success, by taking the Optimal Path to it!
Sometimes it can seem like a Long and Winding Road.
Be aware sometimes the problem.
Is in the Material.
Looks to me like the drop fills shrunk and polishing compound got trapped. They are so small, I would just leave them alone, unless they are going to a customer.
Here' s a Dupont Paint Application Troubleshooting Guide.
All the possible causes for encountering Pinhole Problems are on Page 26 and Solvent Pop on page 30.
Although I thought it obvious, I wondered if I should have explained that Oil and Condensation mixing within in the Compressor are the Major Reason for Material Contamination in Small Workshops.
Oily Water Droplets would be a problem for the material to Adhere to thus a Cause of the Initial Problem. This usually Occurs in the Storage Tank which is why they need to be drained daily. If there is subsequently Insufficient Filtration and Trapping (typically supplied small Compressor Filters are very poor at this) then Oily Moisture is transferred to the work in very tiny quantities. Charcoal Filters are best, but they too should be drained daily and should have a tap at the bottom for this like the Compressor Storage Tank. The bottom line is use the best Filters and Traps you can get and perform these routine tasks once or twice before Spraying.
If you are using a supposedly Oil-less Compressor, then Suspect Solvent Pop as a First Cause.
Its worth Downloading it and Printing the Manual to keep as a Guide.
Good Luck with your Endeavours!