In the November 2007 issue, we reviewed a big batch of waterline stain removers/hull cleaners. Spray Nine’s gel got the nod over the 21 other products, but there were a number of Excellent results—and that’s because these acid-based cleaners don’t mess around.

In a head-to-head test, we applied MaryKate’s On&Off hull cleaner and Rydlyme Marine’s descaler to a hard dinghy’s stained bottom. The MaryKate was faster at cleaning the hull, but its harsh nature is a significant drawback.

One of the most aggressive cleaners out there is the MaryKate On&Off, which received an Excellent rating and was a Recommended product. Made by CRC Industries, it is a caustic stew of acids, mostly hydrochloric (20 to 25 percent by weight) and phosphoric (5 to 10 percent), with a bit of oxalic (1 to 5 percent). We’ve used it often, with good results, but it only would be…