WHAT IS FUEL POLISHING?

Fuel polishing is a maintenance process that cleans stored fuel, most commonly diesel, by continuously circulating it through specialized filtration and separation equipment and returning the cleaned fuel to the tank.

What it does:

Fuel polishing removes contaminants that naturally accumulate in storage tanks over time, including:

• Water (from condensation or ingress)
• Sludge and sediment (rust, dirt, asphaltenes)
• Microbial growth (bacteria and fungi that live at the fuel–water interface)
   

How it works:

Suction from the tank: 

• Fuel is drawn from the tank, typically near the bottom where contamination is heaviest.

Multi-stage treatment:

• Water separation removes free and emulsified water.
• Particulate filtration captures solids down to fine micron levels.
• Optional conditioning (biocides or stabilizers) may be added if needed.

Return to tank: 

• The cleaned fuel is returned—often to the top or opposite end of the tank—to promote circulation and stratification control.
   

Why it’s important:

• Prevents equipment failure in generators, boilers, and engines
• Extends fuel storage life
• Improves reliability for critical systems (e.g., standby power)
• Reduces clogged filters and injector damage
• Fuel polishing vs. simple filtering
• Filtering cleans fuel once, usually during transfer.
• Fuel polishing is recirculating and comprehensive, cleaning the entire tank volume and addressing water and microbial contamination—not just particulates.

Managing expectations:

Access is key! A baffled tank with multiple “compartments” and access to only one section can make it difficult to resolve an existing problem. That is why proactive maintenance and fuel treating are key to a reliable fuel source.

Fuel polishing is a defined process, but it's not magic. We can remove water and debris, but we can't make old fuel, new fuel! 

Diesel fuel shelf life (how long diesel lasts)

Typical shelf life:

• Ultra-low sulfur diesel (ULSD), untreated: 6–12 months
• Diesel with stabilizer (and good storage): 12–24 months
• Well-maintained bulk storage (monitored & polished): 2–5+ years

Diesel doesn’t “expire” on a date—it degrades based on storage conditions and contamination control.

Why diesel degrades?

Diesel problems are usually about contamination, not evaporation:

• Water accumulation (condensation, ingress)
• Microbial growth (bacteria/fungi at the fuel–water interface)
• Oxidation (forms gums and sediments)
• Sediment & rust from tanks and piping

ULSD is more prone to microbial growth than older high-sulfur diesel.

Signs diesel has gone bad:

• Dark, cloudy fuel or visible haze
• Sludge or sediment at the bottom
• Strong sour/rotten odor
• Frequent filter plugging
• Engine issues (hard starts, loss of power)

How to extend diesel shelf life:

• Keep fuel clean, cool, and dry
• Drain water bottoms regularly
• Use a fuel stabilizer for long storage
• Apply biocide only if microbes are present
• Store in sealed tanks
• Test annually (especially for generators)
• Use fuel polishing for tanks storing fuel >12 months

Practical guidance by use:

• Vehicles / equipment: rotate fuel every 6–12 months
• Standby generators (hospitals, data centers):
      Test at least annually; polish or replace fuel if degradation is detected

Gasoline shelf life (how long gas lasts)

Typical shelf life:

• Regular gasoline (no stabilizer): ~3-6 months 
• Ethanol-free gasoline: ~6-12 months 
• Gasoline with fuel stabilizer: up to 12-24 months (sometimes longer under ideal storage) 

Why gasoline goes bad?

Gasoline degrades mainly due to:

• Oxidation: reacts with oxygen, forming gums and varnish 
• Evaporation: lighter components vaporize, reducing octane 
• Ethanol issues (E10): absorbs moisture, leading to phase separation 
• Contamination: dirt, water, and corrosion from containers 

Signs gasoline has degraded:

• Sour or varnish-like odor 
• Darkened or cloudy appearance 
• Hard starting, rough idle, knocking 
• Clogged filters or carburetors 

How to maximize gasoline shelf life:

• Use ethanol-free fuel when possible 
• Add a fuel stabilizer at purchase 
• Store in airtight, approved containers 
• Keep containers cool, dry, and out of sunlight 
• Minimize air space in containers 
• Avoid long-term storage in vented equipment tanks 

Practical guidance:

• For cars and small engines: rotate fuel every 3-6 months 
• For emergency storage: stabilized, ethanol-free gas is best. 
• If fuel is older than a year and unstabilized, it should likely be disposed of.

Bottom Line:

Be proactive about your stored fuel.  Aside from the disruption, potential costly repairs, and a potential lack of compliance, there is a significant financial investment in bulk fuel that if not maintained will become worthless.  Incorporate routine fuel polishing, testing, treatment, and maintain properly functioning tank equipment.  All on your own timeline.

Don't let the lack of a proper fuel maintenance program force you into a reactive position.