What is the issue?It is easy to get rust stains on a seagoing yacht. When carrying out work such as cutting an anchor chain link off, or bolts and shackles, fine metal particles can get left on the boat. Likewise, it is easy to accidentally leave a piece of ferrous metal on deck and come back to see it rusting. The worst, and probably the most common culprit, is taking up rusty old mooring chains and having them sit belay at the stem for a while.
All of these will quickly cause rust marks or an ochre discolouration across a wide area that bonds with the gel coat. Once that happens the discolouration is virtually impossible to remove by normal cleaning methods.
Why address this?Rust stains and the wider ochre discolouration they cause just looks dirty and ugly.
How to address this?Use low pH acids, such as Hydrofloric, Phosphoric, Oxalic, or Muratic acid, to dissolve the rust without the least effort. These acid products are widely available and cost efficient because they are broadly used as household cleaning products.
Acid cleaners are classified by their pH (potential of hydrogen) level ranging from 1.0 to 5.5 with the lower number meaning a stronger acid or higher concentration. The scale goes from 0 to 14. Right in the middle is 7, considered to be neutral. Anything below 7 is acidic. If you are in a hurry you could even try Coca-Cola, pH of 2.52, containing phosphoric acid, or fresh squeezed Lemon, contains 5% to 6% citric acid, that has an average pH of 2.3.
Sailors tend to use Oxalic acid wash, pH of 1.3, but they all function the same. There are many popular marine cleaning products available from chandlers, such as Y10, Star Brite Hull Cleaner, Silky Marine Bright and Chine Shine, that are an oxalic acid of one sort or another. Likewise the popular Barkeepers Friend stain remover. Bought in its natural form as a powder, from most household hardware and marine-supply outlets, it is much better value and allows some experimentation with the concentrate as described below.
The word acid may sound daunting, but oxalic acid is easy to use and totally safe for use about the home and boat. However, it should be treated with respect as it is both poisonous and an irritant. Before you start it is important to read and follow safety advice given on the acid container because the chemical is highly corrosive and also reactive. Protective gloves (e.g., latex), eye goggles, shoes, and thick protective long sleeved clothing should be worn. It gives off a gas that is dangerous so all mixing and application should be in well-ventilated areas. Direct contact can cause chemical burns and damage to a surface. The acid can be neutralized with a base, such as sodium bicarbonate, and then rinsed away using a copious volume of water.
In application, paint or sponge on a five percent solution of oxalic acid to the area and leave it to work for at least five minutes. The oxalic acid should digest the rust and turn it into a white powder that can be brushed off. It will also remove tannins - that rusty orange discolouration you get from the ocean over time that attaches to the gelcoat - and bring back the white of thr gelcoat. Thoroughly rinse off the applied area afterwards. A spray of an alkaline based general household cleaner helps neutralize the acid while rinsing. This you will see, works like magic.
If a stain is particularly difficult and set well into the paintwork try several reapplications. This should work for almost all cases but if you are still not able to lift the stain you can also try a poultice approach. Add a small amount of water to dissolve the crystals and then add talcum powder to create a paste. Apply this poultice to the stain and leave it to dry. Then rinse off again as described above. This should resolve almost any problem.
Oxalic acid solutions can also be used to bleach woods that have turned grey from prolonged exposure to UV light. It is also an excellent deck brightener removing dirt, grime, tannin, fungi as well as rust stains from the surfaces. Personally, before I clean the boat I give the yacht a seasonal wash down using an Oxalic Acid based wash to remove the grime - it is much gentler on the gel coat than the scrubber, and my patience.
One final point that is worth mentioning in passing is, if you do have to cut metal on the deck and have covered it as best you can to trap the particles, a final sweep with a magnet including to the channels taken by any rinse down water, may solve this problem.
With thanks to:Michael Harpur, Yacht Obsession
Removing Rust Stains from Boat Gelcoat using Muriatic Acid
Fiberglass boat hull cleaning with oxalic acid
An extreme case of rust removal with Muriatic acid
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