Pvillefball421 BLUFF:
Barnacle Issue- Try a test area: Soak a dollar store wash cloth in white vinegar. Apply to DRY barnacles. Let it sit for a few hours, but don't let it dry out. Then hit them with a stiff plastic brush and/or that pressure washer of your. Warm vinegar works technically works faster.
Bow Seats: Be careful what grade/ quality of plywood you use. I have seen threads where (after time) the chemicals in regular plywood corroded holes through the transom sheeting.
I do not know if the left over barnacle rings are on bare or painted areas of the aluminum, but I understand not wanting to etch or otherwise discolor the aluminum with commercial acids/ cleaners. They do make cleaners specifically for aluminum so you wouldn't have to worry about ruining the metal. At the end of the day though, barnacle shells are just calcium carbonate (calcite). If you have time, and large amounts of vinegar, you can remove any remaining shells organically. The only risk here is turning your boat into a pickle. I use vinegar often on old fishing reels I find and it works wonders. I know you won't be able to find enough vinegar and a vat large enough to float the boat in though. The tough part of using vinegar is ensuring contact for a LONG time. If you can't flip the hull upside down, the cloth should increase contact time.
