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Old 11-01-2017, 09:28   #1
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Little rust spots on top deck

1) I am on a 2011 Lagoon 400 in the Caribbean. It has a bunch of little rust spots on the fiberglass. Are there metals in seawater that oxidize? What causes theses little spots?

2) The only way to remove them seems to be with a metal bristle brush. Does this hurt the fiberglass? Is there a better way to remove them?
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Old 11-01-2017, 09:32   #2
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Re: Little rust spots on top deck

Someone has been using an angle grinder near the boat in the past and now the shards are rusting. If they were hot when they hit the GRP they melt into the surface. Lots of elbow grease required I am afraid.

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Old 11-01-2017, 09:53   #3
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Re: Little rust spots on top deck

I've had the same issue and used acid to remove them. They eventually rusted again but there were fewer of them. By the third treatment they were all gone.
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Old 11-01-2017, 10:46   #4
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Re: Little rust spots on top deck

Oxalic acid removes rust stains. Doesn't even harm cotton fabric.
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Old 11-01-2017, 11:48   #5
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Re: Little rust spots on top deck

Linda,

I see you're in the Caribbean. Maybe what would work is for the boat owner to find out if FSR is available there. It is an oxalic acid based fiberglass stain remover, and should eventually take care of the problem. Or, the owner may be able to source oxalic acid crystals, which you mix with water in the recommended proportions, and wipe on the area with the stain. Works great on surface stain on s/s stanchions, as well.

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Old 11-01-2017, 12:59   #6
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Re: Little rust spots on top deck

I have found that phosphoric acid works better than oxalic acid. It easy to purchase as it is a common rust remover. In the US one of the brand names is Ospho. It's sold at most hardware stores, it is really only a 20% solution of phosphoric acid. I have had a lot of problems with rusty anchor chains recently staining my deck. (I haven't had a new chain go three months without rusting, galvanized chain is not what it used to be, even name brands) I dilute it 3 parts water to 1 part Ospho and use it as a deck wash. After that I have to use sun glasses to even look at my deck in daylight. Krud Kutter the must for rust, is also 20% phosphoric acid and is cheaper than Ospho. Be careful as Krud Kutter sells several types of cleaners in the same style bottles, make sure it says "The must for rust" on the bottle. Home Depot in the US sells it as driveway cleaner by the gallon at about half the price of a quart of Ospho. Just read the labels and find something that says 20% phosphoric acid as I don't know what brands will be available where you are. The one caveat I will tell you is that if you spot clean the bad stains you will probably find a bright white spot there and the rest of what you thought was a clean deck will look dingy. No scrubbing is required, let it sit 30 minutes and rinse off with water. On really bad stains it may take 2 treatments.
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Old 12-01-2017, 08:08   #7
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Re: Little rust spots on top deck

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Originally Posted by Pete7 View Post
Someone has been using an angle grinder near the boat in the past and now the shards are rusting. If they were hot when they hit the GRP they melt into the surface. Lots of elbow grease required I am afraid.

Pete
Had the same thing happen in a shipyard didn't notice till a month out.
But did remember at the time they were grinding the deck on a tug near by at the time
It did a big number on the stainless on our boat The only way to fix is clean it with a green plastic pad and acid
Once finished i painted it with a can of clear coat its been 6 years now and still shinny.Did the same on the hull.
I was lucky not to get any or much on the deck
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Old 12-01-2017, 08:26   #8
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Re: Little rust spots on top deck

Don't use a wire brush - you will embed more metal fragments into the deck. As already recommended, apply hardware store rust remover, sold to remove stains on walls and patios, caused by iron-loaded water in sprinkler systems. It's cheap, gentle and effective. I usually spray it with a pump up garden sprayer.

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Old 12-01-2017, 08:27   #9
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Re: Little rust spots on top deck

I am a big fan of Barkeeper's Friend. It is readily available at most hardware stores, contains oxalic acid and does a great job of removing rust stains from metal and fiberglass.

Make a paste with water and let it sit on the stain for a few minutes.

It's also great for cleaning Corian sinks and counters.
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Old 12-01-2017, 09:53   #10
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Re: Little rust spots on top deck

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Originally Posted by Captain Bill View Post
I have found that phosphoric acid works better than oxalic acid. It easy to purchase as it is a common rust remover. In the US one of the brand names is Ospho. It's sold at most hardware stores, it is really only a 20% solution of phosphoric acid. I have had a lot of problems with rusty anchor chains recently staining my deck. (I haven't had a new chain go three months without rusting, galvanized chain is not what it used to be, even name brands) I dilute it 3 parts water to 1 part Ospho and use it as a deck wash. After that I have to use sun glasses to even look at my deck in daylight. Krud Kutter the must for rust, is also 20% phosphoric acid and is cheaper than Ospho. Be careful as Krud Kutter sells several types of cleaners in the same style bottles, make sure it says "The must for rust" on the bottle. Home Depot in the US sells it as driveway cleaner by the gallon at about half the price of a quart of Ospho. Just read the labels and find something that says 20% phosphoric acid as I don't know what brands will be available where you are. The one caveat I will tell you is that if you spot clean the bad stains you will probably find a bright white spot there and the rest of what you thought was a clean deck will look dingy. No scrubbing is required, let it sit 30 minutes and rinse off with water. On really bad stains it may take 2 treatments.
Thanks - I've never tried this before. I use Barkeeper's friend, but even with that, some rust stains take a lot of working on to be removed.

Do you have any idea whether phosphoric acid is noticeably harmful to fiberglass or gel coat?

I've never had a bad experience with Barkeeper's friend, but the concern has been there.
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Old 12-01-2017, 09:57   #11
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Re: Little rust spots on top deck

Many toilet bowl cleaners are also 20% phosphoric acid concoctions and,might be less expensive than BOAT stuff. Be sure read the label and avoid hydrochloric based products.
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Old 12-01-2017, 10:08   #12
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Re: Little rust spots on top deck

Oxalic acid comes in many products including bar keepers friend and on/off. The cheapest and easiest way I have found it, is to buy wood bleach from the local hardware store. (Make sure it's oxalic acid). You mix the crystals with water, brush it on the rust and after five minutes just hose it off and it's gone. For heavier stains around stanchions and the like, after five minutes just scrub it a little with the brush and you're done. Cheap, effective and easy.
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Old 12-01-2017, 10:13   #13
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pirate Re: Little rust spots on top deck

Seawater drying and crystalizing on the decks can cause this if you have a long trip with no rain to rinse it off.. most often found in the non slip..
Treatments mentioned above.. I use an acid solution called Ferro net.
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Old 12-01-2017, 11:36   #14
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Re: Little rust spots on top deck

1) A metal brush will do far more harm than good, especially if it has steel bristles. Get rid of the brush.

2) Barkeepers friend has acid but it's also abrasive. That's not good for gelcoat.

3) Pretty much any "rust remover" from the hardware store or even grocery store should work. Let it soak a few minutes, don't start rubbing right away. "whink" is cheap and available at grocery stores, Walmart, home centers, etc.

4) As mentioned above, wood bleach is usually oxalic acid and works well on rust and the brown stains from some river water.

5) "Hull Cleaner" (made for boats) will get rid of rust and you may already have some.

Whatever you try, it should be labeled as "safe for fiberglass". Toilet bowls are typically porcelain and toilet bowl cleaners may not be safe for fiberglass.
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Old 12-01-2017, 11:59   #15
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Re: Little rust spots on top deck

Quote:
Originally Posted by Hardhead View Post
Thanks - I've never tried this before. I use Barkeeper's friend, but even with that, some rust stains take a lot of working on to be removed.

Do you have any idea whether phosphoric acid is noticeably harmful to fiberglass or gel coat?

I've never had a bad experience with Barkeeper's friend, but the concern has been there.
I have been using it for 6 years and have noticed no problems with the gel coat other than the already mentioned blinding whiteness. Once I was in a boat yard and the yard was grinding the red bottom paint off a boat while the wind was blowing in my general direction. I was away from the boat that afternoon and did not notice it when I got back after dark. The dew settled on my deck and the dust turned into a reddish stain which no conventional cleaners would remove. Someone finally suggested I try the Krud Kutter rust remover. That took the stain completely off and restored the deck to a bright white. It also works well for removing the ICW stain though I have found it sometimes changes the color of bottom paint. I use a sponge to control the application of the sides of my boat where I use it to remove rust stains running down from my "Stainless steel" ports. I also treat the ports themselves and it seems to stop them from dripping more rust stains for about 6 months. I typically pressure wash my deck and hull before applying the phosphoric acid. I'm often convinced I don't need to do the whole deck because it's already so white, but after I treat the rust stained areas they are so much whiter than the "clean" rest of the boat I end up doing the whole thing.
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