RE: How to clean my ISonic
it is a common problem. The paint turns yellow with time. I solved this on my Sonic cleaning it with... ash. Well, it was just a try, but it turned out nicely. I used a wet old dish sponge, on its soft side, and ash from the wood stove. I guess ash acts as a mild abrasive; it takes some work to clean, but one does not risk to damage the board.
Probably there are more professional methods also, but this worked anyhow and I will use it again if needed. Plus, it is perfectly ambientally safe.