
I have used three ounce tacks all my career. Into wood and into plastic. I drive the heads solid to the leather. I can pop tacks nearly as fast as I can drive them...but most of my students have as much trouble doing that as any technique in the process. I can understand the frustration.
Personally, I think mould is is more damaging than the black marks left by the tacks. The black marks are nothing more than an iron stain--the chemistry of which is the foundation for "iron black"--a primitive but highly effective black dye made by soaking iron tacks (or steel wool) in vinegar. I have used iron black on a number of occasions and while I cannot guarantee that it is not damaging the leather, I have not witnessed it on articles I have made for my own use.
That said, when iron rusts...a little further on from the initial reaction of the iron with the tannins in the leather...heat is generated. Quite a lot actually. And heat will damage the leather.
Wrapping an insole in rubber or cloth risks encouraging mould. Especially in wet climates. A fan or gentle heat might ameliorate that problem but barring that I'd take the tacks or brads any day...just by way of being cautious.
Having said all that, I watched Al Saguto tack up an insole some years ago and he used brads (not blued), IIRC. But he cut the insole with a somewhat liberal margin and drove the brads into the sides of the last. This had the salutary effect of forcing the insole over the edge and over the featherline...modeling the bottom of the last as effectively as any method I've seen...as well as allowing the black spots and holes to be trimmed off as the insole was cut to shape.
Ultimately we are all gonna have three or four black spots on the insole no matter what we do--the tacks/brads that hold the insole in place while we draft the uppers.
I might add that many years ago I walked into a very old hardware store and saw a whole quarter cask oak barrel of what was labeled "lasting pins." I bought ten pounds, as I recall, and by the time I ran out I had moved and the hardware store had closed. The closest thing I have ever been able to come up with is 18 gauge wire brads in inch or inch and a quarter lengths. They will bend prematurely on occasion but are otherwise an admirable substitute for those old "lasting pins." As the years go by I use them more and more in lieu of the three ounce lasting tacks.
Tight Stitches
DWFII--HCC Member