The silver posca pen certainly seems waterproof once dry - electric toothbrush plus soap and water were no match. Rubbing alcohol and the electric toothbrush quicky removed it though. Interestingly, after the cleanup the insert digits in the V8S seemed brighter than I remembered. So... I decided I'd try the alcohol treatment on the RXF. I'd tried soap and water before to no success, but was a bit wary of removing the factory paint so didn't try anything stronger. The result is really good. Think I'm going to stick with this for the time being.
Before (left is v8s + paint, right stock rxf:
After cleaning both with alcohol + electric toothbrush (same order):
Both photos taken in a room without direct natural light (only an LED light, 40W equivalent at a guess).
Really shines now as soon as you get any direct light (down/spot lights) or natural light (even UK, where it's going dark already at 2pm):