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Old 11-06-2019, 19:52   #1
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Sludge rust at the bottom of my steel fuel tank

Hi all,

I clean my fuel tank yesterday and there was about 5mm layer of redish powder at the bottom of my tank. So it basically contaminates my fuel.

It appears that this is due to the top part of my tank which is rusty (Mild steel) due to the condensation and rust drops on regular basis. I precise that this is how I bought my boat so I presume that the tank was never kept full or sometimes not. So this is the way it is now.

So I was wondering if there is anything I could prevent from happening knowing I can't remove the tank from the boat.

How shall I deal with it?

Thanks.
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Old 11-06-2019, 22:13   #2
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Re: Sludge rust at the bottom of my steel fuel tank

Until last week I had a 1956 David Brown tractor which I bought 30 years ago. The tanks (Petrol and Power Kerosene) were in bad shape so I coated the inside of of one of them with epoxy and ran the tractor on petrol without any problems.

Just now I wanted to know if what I had done had been tried by anyone else and during my research I came across this.

Damon Industries Red-Kote Gas Tank Liner


Maybe the way to go?
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Old 11-06-2019, 22:51   #3
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Re: Sludge rust at the bottom of my steel fuel tank

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Originally Posted by coopec43 View Post
Until last week I had a 1956 David Brown tractor which I bought 30 years ago. The tanks (Petrol and Power Kerosene) were in bad shape so I coated the inside of of one of them with epoxy and ran the tractor on petrol without any problems.

Just now I wanted to know if what I had done had been tried by anyone else and during my research I came across this.

Damon Industries Red-Kote Gas Tank Liner


Maybe the way to go?
I heard of Red-Kote. It seems to a liquid so how could I apply it on the top part of the tank. With a brush maybe?
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Old 12-06-2019, 00:26   #4
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Re: Sludge rust at the bottom of my steel fuel tank

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Originally Posted by zeseb View Post
I heard of Red-Kote. It seems to a liquid so how could I apply it on the top part of the tank. With a brush maybe?

They'd have the answer for that surely.



With my tank I put it in the boot of my car for a week with some heavy ball bearings to roll around: that got rid of the surface rust.


I think you need to do some research as I'm sure it is a common problem. If you do a search on something like "rust fuel tank phosphoric acid" there is a stack of information there.


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Old 12-06-2019, 00:38   #5
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Re: Sludge rust at the bottom of my steel fuel tank

Is the tank rusting due to condensation or water entering the tank? Mainsail who produces this website did a test over a winter with a fuel tank inside a garage and a single pipe to the outside. He found no measurable condensation inside the tank after 6 winter months.

https://marinehowto.com/

If you get rid of the water it won't rust. Are the filler cap seals in good condition? next having cleaned it out keep the tank full. Were is the breather pipe, in an exposed location? could it be moved or some protection added? Interestingly coded boats for commercial work in the UK require a valve to close off the breather pipe to a fuel tank.

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I heard of Red-Kote. It seems to a liquid so how could I apply it on the top part of the tank. With a brush maybe?
If the tank has had diesel in it and you can't remove it from the boat, then I am doubtful you could clean it well enough to get anything to stick to the inside of the tank.
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Old 12-06-2019, 05:07   #6
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Sludge rust at the bottom of my steel fuel tank

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Originally Posted by Pete7 View Post
Is the tank rusting due to condensation or water entering the tank? Mainsail who produces this website did a test over a winter with a fuel tank inside a garage and a single pipe to the outside. He found no measurable condensation inside the tank after 6 winter months.

https://marinehowto.com/


Yes but it had no fuel in it. He did prove that an empty tank wonít condense water, but anyone who has ever left an empty gas can with the lid off in their garage knows that.
What happens is of course during temp changes the tank will sort of inhale and exhale, but water vapor doesnít condense.
Make the tank colder than the atmosphere itís inhaling and it may though. His tank I believe was kept at atmospheric temperature, my fuel tank in my boat is more likely to be at or close to the water temp my boat is sitting in.

Now add fuel to the tank, itís still inhaling and exhaling, but fuel absorbs water, and water can and will come out of suspension and now you have water in your fuel tank.

So he is correct, he was testing to see if a fuel tank will condense water, and it doesnít, but that doesnít mean that we canít get water in our fuel tanks from the atmosphere unfortunately.
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Old 12-06-2019, 05:27   #7
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Re: Sludge rust at the bottom of my steel fuel tank

Your biggest threat may be water in the gas, whether from condensation or another source. Condensation is possible, because the fuel itself is a heat sink, such that the temperature in the tank and the temperature in the open air are different. Step one is find the source. If you cannot eliminate it, then manage it. Step two is a good fuel filter and small amounts of alcohol in your fuel, assuming that your various O rings and gaskets are modern enough to not be disolved by alcohol. Alcohol dissolves readily in both gasoline and water, so the water goes on through your engine.

I've got a question in my mind about the rust coming from the top of the tank. Was this tank allowed to sit empty, or partially full, with lots of air available? Sloshing fuel should stop rust, and gasoline vapor, being more dense than air, should fill the tank, eliminating the air needed for rust. I don't know the answer here, but you might give further thought to just how this situation arose. As a point of reference, my two ordinary steel diesel tanks have not produced a bit of rust in seven years on salt water.
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Old 12-06-2019, 05:39   #8
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Re: Sludge rust at the bottom of my steel fuel tank

once it starts rusting it probably will continue. you could put in some big inspection ports and try to seal the top with the red coat. difficult but worth a try.
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Old 12-06-2019, 05:59   #9
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Re: Sludge rust at the bottom of my steel fuel tank

I had a 36 year old steel tank in my old boat, it plagued me for years. The rust and rust flakes would clog the fuel pickup and stall the engine. I did my best to empty the tank and refill it a number of times, but considering the only inspection hole to work through was the fuel filler gauge, I could never fully clean or coat it.

I gave up and had a new tank made. It's aluminum. It's gorgeous. And designed (by me) for a sailboat, instead of a power boat. Since I sip about 1.3L/h, or 1.5L/h including contingency, I trimmed the size from 46L to 30L (about 20 hours of motoring). I ensured there was an inspection hatch, a separate pickup, vent and return as well as fuel gauge. I went with a vertical design so that when the boat is heeled or rolling, the pickup will still function. The only thing I considered after that I might have liked was to add a secondary pickup that could be used for attaching an external fuel-polisher.



I have no affiliation with Boyd Welding (Florida) who manufactured this tank, but I admire the craftsmanship that went into it. They built it on spec and shipped to me in Canada for a reasonable price. How much is your time worth cleaning, coating, and living with stressful fuel problems vs. installing a rust-proof tank?
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Old 12-06-2019, 08:52   #10
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Re: Sludge rust at the bottom of my steel fuel tank

Having had boats with steel, stainless steel, moulded plastic, aluminum and fiberglass I can say hands down for diesel fuel my preference is fiberglass or rotomolded plastic. As any airplane pilot will tell you there is a reason why part of the pre flight walk around is to drain each and every fuel tank into a little clear tube and inspect for water condensation. It does occur especially in metal tanks.
My experience is that metal tanks sweat considerably more than plastic formulations.
Toto molded tanks are fine but offer limited serviceability or modification as nothing sticks to it reliably. Fiberglass is lightweight. Can be molded into any shape. Is repairable. And can often be molded directly into the hull using the sides or bottom of the boat as part of the structure. Also they are non galvanic and do not propitiate electrical current.
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Old 12-06-2019, 09:08   #11
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Re: Sludge rust at the bottom of my steel fuel tank

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Originally Posted by Squanderbucks View Post
Having had boats with steel, stainless steel, moulded plastic, aluminum and fiberglass I can say hands down for diesel fuel my preference is fiberglass or rotomolded plastic. As any airplane pilot will tell you there is a reason why part of the pre flight walk around is to drain each and every fuel tank into a little clear tube and inspect for water condensation. It does occur especially in metal tanks.
My experience is that metal tanks sweat considerably more than plastic formulations.
Toto molded tanks are fine but offer limited serviceability or modification as nothing sticks to it reliably. Fiberglass is lightweight. Can be molded into any shape. Is repairable. And can often be molded directly into the hull using the sides or bottom of the boat as part of the structure. Also they are non galvanic and do not propitiate electrical current.
Yeah, plastic of some sort is great. I have had two aluminum tanks go bad. One SS tank go bad. All of those went bad where they sit on something. Moisture stays in the gap and corrosion eats the metal.
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Old 12-06-2019, 10:21   #12
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Re: Sludge rust at the bottom of my steel fuel tank

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Moisture stays in the gap and corrosion eats the metal.
Hmmmm... interesting. This is in my cockpit locker, but you make a good point. Perhaps I'll paint the bottom and a few inches up around the base to help prevent corrosion from latent moisture round the tank.

(if the water is high enough to fill my cockpit locker and sit around the tank, I'm in a lot more trouble and won't be worrying about corrosion...)
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Old 12-06-2019, 10:43   #13
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Re: Sludge rust at the bottom of my steel fuel tank

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Hmmmm... interesting. This is in my cockpit locker, but you make a good point. Perhaps I'll paint the bottom and a few inches up around the base to help prevent corrosion from latent moisture round the tank.

(if the water is high enough to fill my cockpit locker and sit around the tank, I'm in a lot more trouble and won't be worrying about corrosion...)
May be a good idea. There are special primers for aluminum, Zinc Chromate I think. Put where the supports are at least. But you will probably be ok there. All my bad tanks were under the floor boards! I replaced the SS one with Titanium! (went bad at 6-7 years! Had a hole the size of a pencil eraser!)
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Old 12-06-2019, 11:25   #14
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Re: Sludge rust at the bottom of my steel fuel tank

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I have no affiliation with Boyd Welding (Florida) who manufactured this tank, but I admire the craftsmanship that went into it.

Boyd made me a custom tank too which I picked up in Ocala. I put a 4" inspection port in myself but yours looks really good
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Old 12-06-2019, 17:23   #15
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Re: Sludge rust at the bottom of my steel fuel tank

Water (condensation) sinks to bottom of tank and provides oxygen for rust, bacteria, and what ever else is nasty. You need a fuel polisher.
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