I did this several times on used balls - and I do not think that there is something illegal about it.
Just works with true engravings, not the Brunswick-produced balls where the engravings are very deep and filled with polyester. On others, I recommend scraping off the old color muck either with a steel needle or, more professional, a mini drill like a Dremel. Most engraving colors are very sticky, and the catch dirt easily, much like oil-based painter colors.
For a "refill", I had best success with glossy, water-based acryllic paint and enamel paint (for model kit use). The acryllic paint should be applied with a flexible spade (e. g. rubber), not a brush (much like in real life ball production, it is very effective). Fill the clean engravings several times, wipe away the residue. When all is dry, you can rub the residue off with a wet towel, or gently use a sanding pad (e. g. before you prepare the ball's surface, I'd recommend this just after engravings refreshment).
Recent "victim" is my Pure Hammer - it caught lots of dirt in its engravings, and upon refreshing its surface I decided to add some style with neon green color above the murky white engravings. Worked pretty well, looks cool (no pics available so far)!
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DizzyFugu - Reporting from Germany
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