I honestly hope you can help. Several weeks ago local seagulls had a sushi feast on my boat. The leftovers were large and small and mostly on non-skid. When I got to the boat I cleaned up whatever I could pick up, but the sun-baked stains remained. Ive tried whatever I possibly could including Barkeepers Friend and Total Boat White Knight. They both have oxalic acid as main ingredient. It stayed there for 30-40 minutes, I didn't touch it. Next I got oxalic acid as powder and made my own very strong mix. Again, no major effect. Next, In desperation, I broke out 3000 PSI power washer, which again didn't remove the stains. My marine biologist friend said to try Draino.
Weve not tried Draino, but can recommend a method that has worked very well for us. With any stain, you want to start with the least aggressive cleaner. An ordinary boat soap or phosphate free laundry detergent (diluted as directed) will get most stains if your hull is recently waxed. If that won't work, your next weapon is diluted chlorine bleach.
If bleach fails, move on to acids. We suggest a cleaner containing hydrochloric acid (HCl). You can make your own solution using diluted muriatic acid (30-35 percent HCl) from a pool supply house or hardware store. This will be the least expensive route (see One Bucket Cleaning Kit, PS May 2017.) Dilute with water to get a 3 percent solution of HCl. Pour acid into water-never the opposite-and protect your eyes. You can try a slightly higher concentration, but be careful! The pH is very low and this concoction will quickly corrode aluminum and most other metals.
Your other option is to use an over-the-counter cleaner (marine or otherwise) that is pre-diluted with hydrochloric acid. You have plenty to choose from. One of our top rust-stain cleaners (see Rust Erasers, PS May 2006 online) or waterline stain cleaners would be a good choice (see PS Tests 22 Hull Cleaners, PS November 2007 online). Spray-type cleaners finished top in both tests, but for a vertical surface, the gels can be more effective. Whatever acid you use, don't let it dry, and rinse well with fresh water.
If acids cant remove the stain, we move on to a rubbing/buffing compound, which removes oxidized layers of gel coat that hold the stain. If you have a light-duty rubbing compound or a cleaner-wax, try that. Youll likely need a heavy duty compound, and weve tested several (See Heavy Duty Rubbing Compounds, PS April 2014.) Unless the entire deck could use a buffing-and it sounds like it might-you can apply these to the stain by hand.
This should take care of the stain, but your work is not done, yet. In order to save you trouble the next time, youll want a layer of wax to protect the surface. The simplest, safest protection for decks is a wash-n-wax. You can opt for a special non-skid wax (see Non-skid Waxes, PS October 2006 online), but the wash-n-wax cleaners produce a surface gloss that makes it easy to hose off bird poop, yet is not too slippery. Theres a barge-load of boat soaps that will work (see Boat Soaps for Regular Washdowns, PS January 2013 online). As an added measure, you could apply a spray wax (see Searching for Spray Waxes, PS December 2014). Most are not too slick for a deck, but to be safe wed test first, or reserve these for spots where you don't need traction.
Finally, you should look at bird deterrents. Physical barriers worked best in our test, although some species have an aversion to owls and kites and things that flash. See Do-it-Yourself Bird Deterrents, Inside Practical Sailor blog.
If youre really serious about cleaning check out our four-volume ebook series: Marine Cleaners, the Complete Series, which covers more than you need to know to keep your boat looking her very best. Its available at our online bookstore www.practical-sailor.com/books.