I had the chance to acid wash my "new to me" boat last weekend. Anyone in doubt, believe me it works. I snagged the idea from Don S. and used a commercial grade toilet bowl cleaner (San-teen I believe) from a $6.00 Wal-Mart sprayer.
Before...
After one coat of cleaner + garden hose to wash off...
Total project cost around $13. If only I could acid wash vinyl
Before...

After one coat of cleaner + garden hose to wash off...

Total project cost around $13. If only I could acid wash vinyl