Putting the Acid on Rust Stains

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Putting the Acid on Rust Stains

Nothing worse than seeing a beautifully clean white fibreglass hull marred by tea stains flowing down from stainless steel fittings.

For some reason, that light coloured stain under the bowsprit bolts, or appearing to drip down from stainless screws, or adding an unattractive blemish to removable deck wash drain screws, seems to be the first thing, middle and final thing that catches the eye.

Nice hull, shame about the stains.

But removing them is a lot simpler than many people think.

Yes, you can beaver away with fibreglass polish in a series of ‘wax on, wax off’ moves that would make Mr Miyagi of Karate Kid fame, very proud.

Or you take a short, fast, inexpensive route that will deliver a result that will make everyone proud.

The secret is to get some phosphoric acid on to the job.

Phosphoric acid which is H3PO4 is a weak acid (by chemical definition), that just loves to react chemically with the tea stain which is a form of surface rust.

You can buy it in undiluted one litre plastic bottles and break it down in accord with the manufacturer’s directions, quite often one part phosphoric acid to five parts water, or you can purchase it in an already-diluted, ready to go, 750ml litre ‘spray on’ bottle.

You’ll find it marketed under names such as Septone’s Boat Care series of products where it is described as rust stain remover.

The process is simple – taking care not to get it in your eyes, your mouth, inhaling it or spraying it on sensitive skin, you simply squirt a little on to the affected area.

Let it sit there for five or 10 minutes, then rinse off with a jet of water.

You might like to use a soft sponge, a soft bristle brush, an old toothbrush or a rag to really push it into the grooves and assist penetration. But there’s no need to go hacking or grinding away.

Badly affected areas might require a repeat application.

Again, just rinse off when done. It won’t affect your fibreglass in fact, it will help remove some of the oxidation that might be appearing on the fibreglass.

I saw one client pour a one litre bottle into a six litre air pressure sprayer – the type with a pump handle that you will find in Bunnings for around $6, then spray his entire boat.

He did a side at a time, then the bow, then the transom rinsing it off after each stage.

That one litre bottle allowed him to go over his six and a half metre half cabin boat, twice.

The job was completed in less than two hours and that included removing very stubborn stains from some particularly hard places.

His 10-year-old boat came up a treat – even more so after he then polished his boat the following day.

For those people who have that characteristic dirty, muddy discoloration around their transom, close to their sterndrive leg, phosphoric acid again could be the ideal solution – literally.

In the same way as dealing with stain steel tea staining, just spray it on to the fibreglass and leave it for five to 10 minutes (the worse the marking, the longer you leave it on) and then wipe it with a soft rag before thoroughly rinsing off.

One guy we know thought the staining around the transom of his sterndrive might be caused by water ingress – the fear of having to replace the transom was weighing heavily on his mind. He thought he might be looking at perhaps a $10,000 repair.

Imagine his joy when we suggested first trying to remove the mark with phosphoric acid as a starting point to ascertain whether it was in fact a blemish that originated and was restricted to the outside surface.

Less than $20 later and suddenly, he was back in love with his boat and its shining, immaculate transom.

It often is a good idea to take a spray bottle away with you when you go camping with your boat. Many caravan parks have a lot of trees to provide shade for tents and vans.

They also provide a lot of branches for sap-like drips and fauna which results in bird and animal droppings as well as varieties of flower seeds, pollens and tree leaves which leave marks on a boat parked under them for a day or two.

Again, just a quick squirt, wait five or 10 minutes, perhaps give the affected area a wipe and then wash it off taking care not to send the run-off into any sensitive area from an environmental perspective.

And how environmentally safe is it?

Well, we’re not in a position to give a full environmental assessment, but you will find that phosphoric acid is regularly included in many commercial cola soft drinks, although I’m not suggesting that is there in the same strength as that for cleaning your boat.

As published in Bush ‘n Beach magazine 2019.