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Best Penetrating Oil: 6 Rust Penetrants Tested on Rusted Bolts (Round 2)

Published June 11, 2018 · Updated August 27, 2026

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Six products were run on half-inch wheel lug studs and nuts, each torqued to 80 ft-lb and then oxidised, with four untreated fasteners held back as a control. Every product was applied once, with a dripper rather than a spray can, onto a washer seated at the base of the nut, and given a single hour to work before the nuts were broken loose. That is a deliberately shorter and more realistic regime than the repeated applications over three hours used in the first round, and it changes what the results mean: this is a test of how fast a penetrant moves, not how far it gets given all afternoon.

A second rig measured creep directly. Bolts were threaded into a one-inch block of plexiglass, drilled and tapped, and oxidised, so each fluid's progress down the threads could be watched through the plastic. That test did most of the separating, and it is where Sea Foam Deep Creep and CRC Knock'er Loose pulled ahead. The odd result of the day came from an extra experiment: a little of every product stirred together into a single mixture penetrated nothing at all, and the nut it was applied to — threaded on by hand and never torqued — still took 56.4 ft-lb to break free.

Top Picks

Best Overall

Sea Foam Deep Creep

Purchased for: $11.99

Finished first. Kept more fluid on the nut and threads than the closely matched CRC over the hour, then ran the full length of the bolt in the creep test and pooled underneath, at $11.99.

Runner-Up

CRC Knock'er Loose

Purchased for: $7.99

Finished second. Mostly dried off the washer during the hour, yet still penetrated thoroughly to the bottom of the bolt in the creep test, for $7.99.

Best of the Rest

WD-40 Specialist Rust Release Penetrant

Purchased for: $9.46

Full-length thread penetration. The Specialist rust-release formula carried all the way to the bottom of the bolt in the creep test, matching the podium on that measure, at $9.46.

Cheapest Option

DOT 3 Brake Fluid

Search on Amazon

Purchased for: $2.78

The old shop remedy held its ground: it stayed on the washer without evaporating and still travelled the full length of the thread in the creep test, for a fraction of the price of a dedicated penetrant.

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Results from August 2019

	Name	Purchased for ($)	Link
	DOT 3 Brake Fluid	$2.78	Search on Amazon
Ordinary DOT 3 brake fluid, included to put the long-standing shop remedy up against products built for the job. It stayed put on the washer through the soak without spreading past it or evaporating, and in the plexiglass creep test it travelled the full length of the thread to the bottom of the bolt.
	CRC Knock'er Loose	$7.99	Search on Amazon
Sold to loosen frozen metal parts, dissolve rust and corrosion, and lubricate afterwards. Most of it had dried by the end of the hour, leaving a little at the base of the nut and some creep away from the washer, but it penetrated thoroughly in the creep test and reached the bottom of the bolt. Second overall.
	Sea Foam Deep Creep	$11.99	Search on Amazon
Sold to power through rust and varnish. It behaved much like CRC Knock'er Loose over the hour but kept more fluid on the nut and threads and crept less far off the washer, and in the creep test it ran the full length of the bolt and pooled underneath, on the point of dripping off. The pick for best overall.
	Fluid Film	$12.39	Search on Amazon
A lanolin-based product sold to free up heavily seized parts, and the one that behaved least like the others. It coated the entire nut-and-bolt assembly in a clear film and clung there, but in the creep test it stayed near the top of the threads rather than working its way down.
	BG In-Force	$24.95	Search on Amazon
Sold on an ion-activated formula claimed to permeate metal better than standard penetrating oils, and the most expensive product here. It flowed all the way down the nut and onto the washer and resisted evaporating, but in the creep test it worked only partway down the threads, with much of it still sitting at the top.
	WD-40 Specialist Rust Release Penetrant	$9.46	Search on Amazon
The rust-release penetrant from the Specialist line, a different formula from the original WD-40. In the creep test it carried all the way down to the bottom of the bolt, matching the two podium products on that measure.

Price

Lower is better

Prices ran from $2.78 for a bottle of DOT 3 brake fluid to $24.95 for BG In-Force, a nine-fold spread. The two products that finished on top sit in the middle of it, at $11.99 and $7.99.

The Verdict

Sea Foam Deep Creep is the pick for best overall. Given one hour on a lug nut torqued to 80 ft-lb, it kept more fluid on the nut and threads than the closely matched CRC Knock'er Loose and crept less far off the washer, and in the plexiglass creep test it travelled the entire length of the bolt and gathered in a puddle underneath. CRC Knock'er Loose finished second on the same evidence: it had largely dried off the washer by the end of the hour, but it still worked its way through the rust to the bottom of the bolt. At $11.99 and $7.99 neither is an expensive product, and the $24.95 BG In-Force did not buy anything extra — it flowed readily down the outside of the nut but only worked partway down the threads in the same hour.

For value, the surprise is DOT 3 brake fluid at $2.78. It did not win, but it was one of the products that carried the full length of the thread in the creep test, and it neither evaporated nor wandered off the washer. As a stopgap for a seized fastener when there is nothing else in the cabinet, the testing gives it a fair case. The WD-40 Specialist rust-release penetrant is the other one worth noting outside the podium, reaching the bottom of the bolt for $9.46; it is a different formula from the original WD-40 and behaved like a purpose-built penetrant here.

Two cautions for anyone buying on this. First, the one-hour window is the whole frame of the test — Fluid Film and BG In-Force both coat and cling well, and a longer dwell is a different question from the one asked here; Fluid Film in particular is built as a corrosion-preventive film more than a fast creeper, and it behaved exactly that way, sliming the whole assembly without moving down the threads. Second, do not combine products. A blend of all six, applied to a nut that had only been run on by hand, penetrated nothing and still needed 56.4 ft-lb to break loose. Buy one penetrant, apply it to a clean washer at the base of the nut, and give it time.