I have a Singer 15-91 that I am eager to get all cleaned up, but I decided to test it on my Singer 99-13 first. The decals on the 99-13 were not in the greatest shape to start with--most were already silvered, or missing completely.
Before the decals were dark, dusty, dingy, and covered in a slightly sticky residue.
And here is the after photo. I rubbed these decals a lot, using at least three applications of TR-3 on cotton balls. The good news is, the still golden parts of the decals didn't get any lighter. The bad news, however, is that the parts of the decals that were already silver rubbed away with the grime. The black paint polished up to a mirror like shine.
Since I wanted to be more careful this time with the decals, I used a plastic kitchen scraper to scrape off what I could of the tar, and then cleaned and shined it up with the TR-3.
In some places the residue was a little more stubborn and took a lot more passes to remove. When the TR-3 dried on it, it became the brownish discoloration you see in the photo on the left. However, with enough cotton balls, the TR-3 did remove it, with no ill effects on the black paint.
So here's the moral of this tail. I will repeat what I read on that other website. Test, test, test!
Be very careful on the decals. One or two passes doesn't seem to do any harm. I suspect on a machine not this dirty, or with less stubborn grime, the decals would be just fine. Even so, in the future I will use this stuff on the decals last, after I've polished up the rest of the machine.
Also, assess the mess on your particular machine. You may not be able to get the decals completely clean using the TR-3, at least not without rubbing them completely off, so you'll want to make sure any dirt and grime doesn't discolor too badly. Find an inconspicuous spot on the machine to apply the TR-3, and see what any remaining grime looks like after the polish has had time to dry.
Here's what my Singer 99-13 looks like now . . .
I hardly went over the decals on the front of the machine bed at all, so they look about the same as when I started--silvered and worn away. You can probably see that I've got a bit more cleaning to do on the front, but it will all clean up and polish up nicely tomorrow. I needed a break. :~)