Fuel Polishing - A Complete DIY Guide

Fuel Polishing: A Complete DIY Guide | Life of Sailing

Last Updated by

Daniel Wade

June 15, 2022

Fuel polishing is especially important for boats, as dirty diesel fuel can be a nightmare for boat owners and cause damage to engines.

Fuel polishing systems remove water, bacteria, fungi, dirt, and sludge from marine diesel and boat fuel tanks. Unlike standard filters and water separators, fuel polishing can remove microscopic contaminants and invisible emulsified water from diesel fuel.

In this article, we'll go over the benefits of fuel polishing systems, how they work, and whether or not you need one on your boat. We'll compare them to standard fuel filters and help you choose between these two common options. We'll also list and explain the parts you need to build your own DIY fuel polishing system.

We compiled information for this article from numerous sources on fuel polishing, diesel quality, and manufacturers of filtration systems for marine and land-based use. We also studied the market to find the latest fuel polishing system prices, technology, and quality.


Table of contents

What is Fuel Polishing?

Fuel polishing is a technical term for a complicated fuel decontamination process. Fuel polishing removes numerous contaminants from fuel that standard automotive-type fuel filters don't. Fuel polishing is especially important for marine applications, as fuel stored above ground is prone to contamination.

Standard fuel filters remove large particles and sediment (I.E., dirt) from gasoline or diesel using a membrane, such as paper or mesh. This works well for underground fuel storage, as fuel at gas stations is drained and replenished frequently. For marine applications, particularly diesel, additional steps must be taken to cleanse fuel of impurities.

What Does Fuel Polishing Do?

Fuel polishing removes large particulates (such as dirt) along with water, bacteria, fine particles, and other organic compounds. Here are the primary contaminants that are removed using fuel polishing.

  • Water
  • Dirt
  • Bacteria
  • Emulsified Water
  • Yeast
  • Mold
  • Dissolved Water
  • Rust
  • Wax
  • Organic films
  • Algae
  • Soot
  • Gums
  • Sap

Fuel polishing is used primarily for diesel fuel, as diesel engines are more sensitive to fuel quality than gasoline engines. This is because diesel engines use compression to ignite fuel (as opposed to a spark plug), so any water or other contaminants can severely damage the high-pressure systems.

A small amount of dirt can clog injectors, and water can cause the engine to self-destruct because it doesn't burn away and can't be compressed. Water is the biggest danger to diesel engines, and it's a common contaminant in marine diesel fuel.

How Fuel Polishing Works

A fuel polishing system is usually connected directly to a diesel tank with two separate lines. The intake line, diesel is drawn into the polisher with a pump, is situated with its opening at the lowest possible point in the tank. This is because scum and contaminants tend to settle at the bottom, where the hose can easily pick them up.

Fuel is pumped into the polisher and then back into the tank from the very opposite top relative to the intake. This prevents polished fuel from being drawn back into the polisher.

Stages of Fuel Polishing

The most basic fuel polishing systems contain a minimum of two cleaning apparatuses, known as 'stages.' The first stage of fuel polishing is usually filtration and free water separation. The second stage utilizes a coalescer, which removes emulsified water, which is essentially dissolved moisture.

Free Water Separation

Free water is liquid water that settles inside a fuel tank. It isn't dissolved into the diesel, and it's easy to remove using a centrifugal water separator. Many boats already have a water separator installed, so this process will be familiar to some. The centrifuge works by spinning the solution and separating fuel and water based on their densities.

Centrifugal separators are also useful for filtering out large particles and sludge, such as some of the build-up caused by diesel bug. However, a centrifugal separator cannot remove all contaminants.


Fuel polishing systems filter fuel at least once, usually before entering the coalescer. These fuel filters use a very fine membrane to separate out all particles of dirt, grime, and other gunk to about 25 microns in size. However, filters and centrifugal water separators can't remove emulsified water or some dissolved contaminants from diesel fuel, which is why a coalescer is necessary after filtration.


A fuel polishing coalescer is essentially an advanced filter. It is capable of separating different chemicals based on molecular weight, and it can remove dissolved and emulsified water and other contaminants from diesel fuel.

Some coalescers utilize electrically charged plates, while others rely on purely mechanical processes. Many fuel polishing coalescers also contain a fine filter to remove any remaining particles that escape the first two stages.

Why is Fuel Polishing Necessary for Boats?

Diesel-powered marine engines are robust and reliable. That said, they're still sensitive to fuel contamination, as bad fuel is one of the most common causes of engine failure at sea. Marine diesel engines are victims of a kind of catch-22, as they require more careful attention to fuel, but their fuel is more likely to be contaminated.

Marine diesel tanks get dirty. Shore-based storage tanks, especially at marinas, are often contaminated due to infrequent use or lack of cleaning. Particulates such as bird droppings and rust are particularly common. A quick fuel sample from a local marina revealed that there was a visible amount of dirt in the fuel, and the presence of dissolved water couldn't be ruled out.

A marine fuel-polishing system removes water and other contaminants from fuel. These systems are compact and reliable and an absolute necessity on marine diesel power plants.

Is a Standard Fuel Filter Enough?

Some factors to consider when choosing between a standard filter and a fuel polishing system are the size of your boat, how far you intend to sail, and the size of your fuel tank.

Generally speaking, a standard fuel filter is not enough for a large diesel-powered boat to operate safely. This is due to the high amount of contaminants found in marine diesel fuel. Having a basic fuel filter is still better than having nothing at all, though a robust fuel polishing setup is always the best bet.

If you don't have room on your boat for a complete fuel polishing setup, make sure to install a fuel and water separator. This will ensure that you won't experience hydro-locking, which is a condition where water prevents a cylinder from compressing, and damage occurs. An in-line fuel and water separator is much cheaper than replacing a bent connecting rod.

What is Diesel Bug?

Diesel bug is a rather horrific-looking bacterial or microbial infection that affects diesel and fuel-containing parts. The condition occurs when water permeates the fuel and allows bacteria or fungus to enter and thrive within it. Diesel bug is especially common in biodiesel and in marine diesel tanks.

Diesel bug causes an enormous amount of sludge to build up in hoses, on engine parts, in and around the fuel tank, and even inside of your injection pump.

Does Fuel Polishing Remove Diesel Bug?

Fuel polishing is the best way to prevent diesel bug from migrating to more sensitive parts of your engine from your fuel tank. If you have the option, it's also best to fill your tanks with polished fuel to prevent diesel bug from infecting your tank and clogging filters.

How Much Does a Fuel Polishing System Cost?

Fuel polishing systems are relatively expensive, and the cost isn't justified on all vessels. The most affordable fuel polishing systems cost anywhere between $1,500 and $3,500. Price usually increases as polishing capacity expands.

The most basic fuel polishing systems at entry-level prices use very basic control systems and may require manual input to start and stop. More expensive fuel polishing systems, which typically cost more than $3,000, come with very advanced and reliable automated controls.

High-end fuel polishers are self-regulating and can keep your fuel tank clean for a long time. High-tech sensors monitor the life of filters, the quality of the fuel in your tank, and how long it'll take to polish the remaining fuel.

Do I Need a Fuel Polishing System?

Do all diesel-powered boats need a fuel polishing system, and are fuel polishing systems available for boats of all sizes? The short answer is no. So how do you know if you need a fuel polishing system?

Generally speaking, fuel polishing systems are most effective for vessels with a large fuel capacity. A 200-gallon tank can be scrubbed using an inexpensive fuel tank cleaning machine, whereas a 2000 gallon tank could benefit from a small fuel polisher.

In most cases, diesel-powered boats less than 40 or 50 feet in length probably don't need a fuel system, though small fuel polishers are available for medium-sized boats. In all other cases, a high-quality fuel filter and a centrifugal water separator are good enough to get the job done, and you'll likely be able to solve any contamination issues some other way.

The one notable exception is with ocean-going vessels, even smaller boats less than fifty feet in length. Boats that travel long distances over the open ocean cannot afford to risk a catastrophic failure due to diesel contamination. A fuel polishing system is a mechanical insurance policy against preventable breakdowns at sea.

Can You Build a Fuel Polishing System?

Yes, it's relatively easy to build your own fuel polishing system if you understand the operational principles and acquire all the right parts. The most important things to consider when building your own fuel polisher are proportioning and safety.

The majority of fuel polisher parts are available stand-alone fuel system add-ons. These include centrifugal separators, filters, electric fuel pumps, and coalescers. When acquired separately, these parts cost significantly less than many complete systems.

You can add your homemade fuel polisher to your existing fuel system as well and keep your fuel water separator and original fuel filter. So if you're going to build your own fuel polisher, it may be a good idea to upgrade the entire filtration system.

Can You Rent a Fuel Polisher?

In most cases, you can't just 'rent' a fuel polisher, though you can utilize the services of a fuel cleaning company. There are dozens of fuel cleaning companies located along the coastal United States that specialize in cleaning marine fuel tanks and purifying diesel fuel.

The length of the process and the cost depends on the amount of fuel you need polished and your location. Hiring a fuel polishing company to purify your fuel is an economical way to rid your tank of sludge, diesel bug, and other undesirable and potentially harmful chemicals.

Fuel Polishing - A Complete DIY Guide
Daniel Wade

Daniel Wade

I've personally had thousands of questions about sailing and sailboats over the years. As I learn and experience sailing, and the community, I share the answers that work and make sense to me, here on Life of Sailing.

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