Maybe not so far from the truth; if you want the whole motor to be white? Maybe not so much sunscreen; but has anyone on your boat ever used some form of (Strong deet) bug repellent? Repellent that may have inadvertently got sprayed on or dumped on top of the plate? Deet is an amazing bug repellent; it is also good for starting camp fires (better then gasoline), discoloring painted surfaces and Oh yea causing cancer. A buddy of mine worked for Bell Canada (from the70s till retirement). Back in his early days; the general worker's supply warehouses, had cases of deet, in 4-5 ounce squeeze bottles of all but strait 100% deet.
Long story short. if you got that stuff on anything; it would turn it chalk white with-in minutes. Worked great keeping the flies away; but I made the mistake of wiping my hands (this crap burned) on the tan seat of my boat. After the sun hit it; it looked like it was tie-dyed; it pulled the colour right out of the seat.
Dan.