Fuel tank polishing, explained: one name, two trades, and what the truck's manual says

One name, two trades

The phrase fuel tank polishing is used for two different jobs. One is done to the diesel inside the tank. The other is done to the aluminium on the outside. We do the second: it is exterior work on aluminium, sold as an add-on to a wash. We do not treat fuel.

This post is for the person working out which of the two they need. For each one we went to what truck makers themselves publish: the operator manuals Kenworth, Peterbilt and Freightliner keep on their own websites. The manual that governs your truck is your truck's; what follows is a reading of three.

The inside job: water in the diesel

Kenworth's operator manual for its W900, T660, T800 and C500 models explains the problem in one line: "Air inside the fuel tanks allows water to condense in the tank." Its advice is about fuel level, not cleaning. For a truck parked for an extended period, fill the tanks to 95 per cent of capacity and never more, because the rest is room for expansion. Day to day, keep the tanks at least half full "to reduce condensation of moisture in the tanks", and it adds: "This moisture can damage the engine."

What that water does next is documented for storage tanks rather than for trucks. The Clean Diesel Fuel Alliance guidance on underground diesel storage, written for fuel-dispensing facilities, says less than a quarter of an inch of water is more than sufficient for microbial growth, that the organisms live where the fuel meets the water and feed on the fuel, and that their presence can lead to plugged filters and pump and injector problems. The EPA's question-and-answer sheet on corrosion in underground diesel tanks gives the remedy in the same order: monitor for water and remove it, and, if corrosion is moderate or severe, a servicing company may suggest a microbiocide or a corrosion-inhibitor treatment.

Two things about that. Those documents are about tanks in the ground, and we quote them for what the problem is, not as instructions for a saddle tank. And neither one uses the word polish: we searched both and it does not appear. On the fuel side it is a fuel-servicing trade with its own contractors, and it is not ours. If your trouble is filters plugging or water in the fuel, that is the company to call.

The outside job: what the manuals ask for

On the outside of the truck the same Kenworth manual is short and plain. Under Chrome and Aluminum Surfaces it says: "Clean aluminum wheels and bumpers with warm water. Tar remover will get rid of heavy deposits of road grime. To prevent spotting, wipe aluminum surfaces dry after washing." For corrosive conditions such as salted roads it adds steam or high-pressure water, says a mild soap solution will help, and ends on two words: "Rinse thoroughly."

Peterbilt's medium-duty operator manual, dated November 2020, carries the same passage almost word for word, with one change worth noticing: cool water where Kenworth's, dated March 2017, says warm. We are not going to tell you which is right. Two manuals disagree on the temperature and agree on almost everything else: water, a mild soap, a thorough rinse, wiped dry.

Freightliner's Cascadia Driver's Manual comes at it from the finish. Among its guidelines for extending the life of the vehicle's finish: "Avoid washing your vehicle in the hot sun." And before any product goes on: "Carefully read all instructions before using or applying any cleaner or product on the vehicle or components. Failure to follow manufacturers' recommendations can result in damage to the finish."

Here is what we did not find. None of the three cleaning sections names fuel tanks: they speak of aluminium wheels, bumpers, parts and surfaces, and Freightliner's chapter has no aluminium heading at all. None of them mentions an aluminium brightener. So in those sections these makers do not tell you to polish your tanks, and they do not tell you not to. What Kenworth and Peterbilt describe for aluminium is water, a mild soap, a thorough rinse, and wiping it dry.

Why a tank goes dull anyway

If the manual's instruction is that gentle, the question is why polished aluminium dulls, spots and stains. One published answer comes from a wheel maker. Accuride's bulletin for its bare polished wheels says a wheel left without a protective polish, wax or sealant is "susceptible to water spotting and/or pitting from environmental contaminants". The same bulletin says residual cleaning solution can stain aluminium wheels, and that soap must not be allowed to dry on the surface.

Chemistry is the other route. Hydro, the aluminium producer, explains in its design manual that the metal protects itself with a thin oxide layer that repairs itself when it is damaged, and that the layer "is stable in the general pH range 4 – 9. In strongly acid or alkaline environments, aluminium normally corrodes relatively rapidly." Accuride, writing about its polished wheels, puts the consequence in terms an owner will recognise: strong detergents and alkaline or acidic cleaners "can etch the surface of the aluminum and leave dull areas".

And one fluid the truck itself carries is described by a truck maker as mildly corrosive to this metal. Freightliner's manual says diesel exhaust fluid "is mildly corrosive to aluminum, but does not affect the strength or structure of the aluminum", and that the white crystals it can leave around components it touches "are easily removed with water". That is the maker's statement about the fluid. It is not a diagnosis of any mark on your tank.

A brightener is not a polish

A brightener and a polish are not the same thing. A brightener is a chemical. The Centers for Disease Control and Prevention, reporting on injuries in Washington State, wrote that "car and truck wash cleaning products, rust removers, and aluminum brighteners often contain HF because it is efficient in breaking down roadway matter". HF is hydrofluoric acid. The same report notes that two of the products involved contained ammonium bifluoride, "a chemical that dissociates into HF when dissolved in water and therefore has similar toxicity".

That report counted one fatal hydrofluoric acid ingestion and 48 chemical burns from hydrofluoric acid exposure associated with car and truck washing in that one state between 2001 and 2013, and it explains why the hazard is easy to miss: at the concentrations used in vehicle washing, "no initial pain alerts the worker to the exposure". Its conclusion is the useful part for an owner: "Less hazardous alternatives to HF-based wash products are available."

Polish is the other thing. In Accuride's bare-wheel bulletin it is a product worked onto metal that has already been washed, rinsed and dried: a metal cleaner or polish "compatible with aluminum to remove spots, stains, and aluminum oxide/corrosion", followed by wax, with a warning against "overly abrasive polishes". The bulletin is also frank about how long it lasts. Water spotting "may require additional polishing", repeated applications may be necessary, and the wax is to be reapplied on a regular basis. Polishing is upkeep. It is not a finish you buy once.

Coated or bare comes first

What that last paragraph reports is what Accuride says about its bare polished wheels. For its coated wheels it publishes a separate bulletin, and that one reverses the advice: "Do not use polishes or wax". The coating keeps its shine without them. It is also thin. The bulletin's repair procedure puts the finish at two to three thousandths of an inch and warns that if you sand through it "you will dull the base aluminum surface and there is no repair". Alcoa describes its treated wheels as "the no-polish, easy-maintenance way to bright, shiny wheels", bars hydrofluoric, hydrochloric and sulfuric acid on them, wants them cooled below 95°F before cleaning, and tells owners who use "mobile wash systems or major truck wash facilities" to remind them which wheels the truck has.

Those are wheel documents, and we are not going to pretend they are tank documents. The three truck makers' manuals we read do not distinguish coated aluminium from bare at all, and we did not find a truck or tank maker publishing a figure for how many tanks leave the factory coated. What carries over is the order of operations. Before anything is applied, somebody has to know which surface is in front of them, and it is fair to ask how they know.

Five questions before anyone polishes your tanks

Ask these of any contractor before they touch your tanks.

Inside or outside? If the trouble is water in the fuel or filters plugging, you need a fuel-servicing company. If it is how the tanks look, you need the exterior trade.

Coated or bare, and how do you know? If you have the truck's build sheet, or you know what it was ordered with, say so.

What is in the product? Ask for the Safety Data Sheet. The CDC report took its hydrofluoric acid percentages from the products' own Safety Data Sheets, so that is where the information lives.

When will the metal be worked? Freightliner says to avoid washing in the hot sun, and Alcoa wants its wheels below 95°F before cleaning. That is a scheduling question, and it is one reason we typically work nights and weekends.

How is it rinsed and dried? Kenworth: rinse thoroughly, wipe dry. Accuride, for its wheels: do not let soap dry on the surface.

And one limit to hear before you book: polish does not put metal back. Where a strong cleaner has already etched the surface, what is left, in the words Accuride uses about its wheels, is dull areas. We have no source that says how much of that a polish recovers, so we will not put a number on it. It is something to look at on the truck.

We polish the outside of aluminium tanks as an add-on to a wash, at your yard. Estimates are free, in person or by text, and we get back to you within 4 hours.