303 and UV Tech

Do you apply 303 or other UV blocking product to just the top of the boat OR the whole thing?

The whole thing
My boat sometimes has to spend a few hours in the sun on its side or upside-down.

303 on what cooks!
I bag my boat after paddling, but it’s in the water for at least an hour, with the deck baking. I put 303 on my deck (naked carbon/kevlar) & wipe it down before re-bagging. It helps keep it from white-out. My hull looks much darker since it’s in the water. If I have big storms coming, I won’t bag it (bag/boat fill with water and hard to bail) & use 303.