Best penetrating fluids 2026

Seized nuts and bolts are the most frustrating part of any motoring DIY, with the potential to turn a quick job into a mammoth task. Whether caused by overtightening or corrosion, a fastener that refuses to budge will soon become rounded as tools slip off, making it even more difficult to remove. In the worst case the fastener will snap, leaving you with no choice but to reach for the drill or welding torch. 

Soaking any stubborn-looking nuts and screws in penetrating fluid will give you by far the best chance of getting them off cleanly, saving you time and meaning you should be able to reuse the fastenings instead of searching for a replacement. While many all-purpose lubricating sprays will claim to have a penetrating release action, a specialist spray promises to be more effective. We racked up the rusty nuts to test seven at a variety of price points.

How we tested

We assembled six identical nut and bolt combinations that had been covered in salty water and left to rot outside for more than a year. We sprayed the test fluids into the top of the nut and watched how far the liquid tracked down through the thread after 10 minutes. We then measured the torque needed to undo the nuts.

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As not all fastenings are easy to reach, we also used rusty nails placed upright in a pool of the fluid to test its ability to track upwards through corrosion. Finally, we gave points for cans that were easy to use and good value.

Rocket TT

8

• Price: around £5.49 
• Size: 450ml 
• Cost per 50ml: 61p
• Contact: mymotorworld.com
• Rating: 5.0 stars

We had to double check the samples to make sure we’d got the right product because we didn’t expect the all-purpose Rocket TT to perform this well in our tests. It’s pitched as a ‘super lubricant’ rather than penetrating oil, but it reached the highest up our rusty nail and filtered quickly downwards through the nut, too. After a 10-minute soak, it released the nut with a torque as low as the pricey Bulldog’s. Add in the low cost and easy-to-use flexible straw and this Rocket shoots to the top.

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Bulldog BDX 01

8

• Price: around £12.99 
• Size: 400ml
• Cost per 50ml: £1.62
• Contact: lubricantsuppliers.com
• Rating: 4.0 stars

You might never have heard of Bulldog BDX, but some car restorers will refuse to turn a bolt without it. However, we were surprised that it didn’t track as far as rivals up our test nail and took longer to filter down through the nut, too. 

If you let it take its time though, it creates a super-slippery surface which also seemed to dissolve some of the rust, giving the equal lowest removal torque reading here. It’s pricey though, especially if you factor in delivery - so save it for the rustiest nuts.

WD-40 Specialist Penetrant

8

• Price: around £9.48 
• Size: 400ml
• Cost per 50ml: £1.19
• Contact: toolstation.com
• Rating: 4.0 stars

Rather than rely on the regular jack-of-all-trades WD-40, which is found in every shed across the land, the company now has a range of specialist products, including this penetrating fluid. 

It soaked quickly up and through our rusty test samples and then gave the third-lowest torque reading to release the nut.  It also gained points for the best nozzle and straw. However, it’s far from being the cheapest option, and the Rocket TT did more for less money.

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GT85

8

• Price: around £4.00 
• Size: 400ml
• Cost per 50ml: 50p
• Contact: wickes.co.uk
• Rating: 3.5 stars

As with our overall winner, the GT85 product isn’t actually a specialist penetrating fluid and is instead pitched as an all-round lubricant. Despite this it had a decent performance in our seepage tests and certainly helped turn the nut easily, seemingly helping to clear away some of the rust and dirt too. It’s also good value, costing the same as the ‘own brand’ rivals. If there’s a downside, it’s that the basic nozzle-and-straw arrangement is a significant disappointment.

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Halfords Release Spray

8

• Price: £4.99 
• Size: 500ml
• Cost per 50ml: 50p 
• Contact: halfords.com
• Rating: 3.5 stars

The Halfords-branded spray isn’t the cheapest in the test, but your £4.99 buys you a 500ml container of the stuff, which is more than any of the rivals here. Its performance was mid-chart in our tests though, making it difficult to completely recommend. 

There is nothing clever about the nozzle either, but we liked the fact the can would still spray if held upside down – that’s useful when you are trying to reach an awkward nut underneath a car.

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OB41 Multi Use Penetrating Oil

8

• Price: around £4.49 
• Size: 400ml
• Cost per 50ml: 56p 
• Contact: screwfix.com
• Rating: 3.0 stars

There is nothing fundamentally wrong with the OB41 offering. The Multi-Use Penetrating Spray put in a decent performance in the tests, and is comprised of a clear liquid that leaves a protective oily layer on the surface of rusty fastenings to help prevent future corrosion. It can also be used as a lubricant for locks and squeaky hinges. That said, it’s not cheap enough to make it the best budget choice, and the lack of straw is an irritation.

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PlusGas

8

• Price: around £10.49 
• Size: 400ml
• Cost per 50ml: £1.31
• Contact: plusgas.co.uk
• Rating: 3.0 stars

For many, PlusGas is the generic name for penetrating sprays. The viscous fluid seemed watery when compared with our winners and took longer to seep up our rusty nail. It dissolved some rust, but the torque needed to undo the nut was no better than the OB41 and Halfords sprays. The extra cost is difficult to justify.

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Verdict

Rocket TT goes straight to the top of our podium with great performance and a low price. Bulldog BDX could be better if you are facing really rusty nuts, but it’s expensive. Third goes to WD-40’s Specialist Penetrant, which does a good job and is easiest to use, but looks pricey next to our winner.

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