Tannin staining of your hull can be frustrating but luckily it is easy to remove, if you use the right chemicals or product. Any job goes smoother when the right product is chosen over the wrong product. Sadly over the years I have watched number of boat owners destroy gelcoat or paint by using sandpaper or abrasives to try and remove tannin stains. This is not necessary and as a matter of fact no elbow grease is required at all.
While oxalic is dirt cheap and works extremely well for this task, I prefer to use a gelled acid for this task. The gel formulation is an attempt to keep the acid off the anti-fouling bottom paint and keep it on the staining, where it belongs.
Acids can eat the copper right out of the bottom paint if not used carefully. This could potentially make the one or two inches above the waterline basically useless at repelling growth if you cleaned your boat in-water then loaded it for cruising. If you do it out of the water, without a drip skirt you could ruin a lot more of the bottom paint.
The hull of my own boat is always well waxed and I really don't get to much if any waterline staining. A good coat of wax, or a polymer sealer such as Awlcare, can go a long way to minimizing these brown stains. I did however leave my inflatable in the water for a few weeks and got enough staining to illustrate the process.
For Gelcoat, Not Painted Hulls:
This task is easy and is also safe for gelcoated hulls. While I have personally used mild acid formulations on LPU paints such as Awlgrip or Alexseal and Acrylic urethane products such as Awlcraft 2000 or Imron I don't suggest it, nor do the paint manufacturers.
If you must use an acid on paint please be really, really, really, really careful and rinse it off immediately after the stain disappears DO NOT LET IT DRY. Do small 3-4' sections before moving on and rinse, rinse rinse once the stain is gone. For a painted hull use the mildest acid you can find. I like a product called Spotless Stainless, which is citric acid based, not oxalic or hydrochloric based.
To clean waterline stains I use MaryKate On & Off Gel pictured here. This is a huge improvement over the regular non-gel On & Off or basic Oxalic crystals from the hardware store. It's also a lot cheaper and stronger than products like Davis FSR stain remover, but works similarly.
There are a lot of products out there that contain an acid that will work but the On & Off Gel is one of my favorites. It also has much less "acid" odor than the regular On & Off and hits your nose a lot less forcefully... This is a great step to do before waxing a boat and it will whiten up most any old tannin/rust stained gelcoated hull.
Always wear rubber gloves & eye protection! Remember this stuff is acid. I use cheap chip brushes to apply it and toss them when I am done.
Brush It On
This whole process, on my inflatable, takes less than 10 minutes and requires absolutely no rubbing. Simply apply with the chip brush and watch it eat away the stains. Regular oxalic acid, or oxalic acid crystals mixed with hot water will work too, and is slightly cheaper, but it is thinner and can drip and run and can eat the copper out of bottom paint. The gel stays put for the most part with considerably less dripping.
Rinse It Off
Simply brush it on and watch the hull whiten before your eyes. About as simple and effective as it gets. Once clean simply rinse and dilute the acid and you are ready to go. No need to let your boat grow a mustache, and look like a 70's porn star. Now that you know the trick to removing tannin stains your boat will look good for years to come..
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