Best Penetrating Oil for Seized Bolts: Kroil vs. Free All vs. WD-40
Best Penetrating Oil for Seized Bolts: Kroil vs. Free All vs. WD-40
When a bolt won't budge, the right penetrating oil can mean the difference between a 10-minute job and a broken stud. Here's how the three most popular penetrants compare — and which one to reach for.
WD-40
WD-40 is everywhere, but it's not actually a penetrating oil — it's a water displacer and light lubricant. It works fine for light surface rust and squeaky hinges, but it's not the right tool for seriously seized fasteners. The penetrating action is limited compared to dedicated penetrants, and it evaporates quickly without leaving much behind.
Best for: Light rust, moisture displacement, general lubrication
Not ideal for: Heavily corroded or seized fasteners
Free All
Free All is a professional-grade penetrating oil that outperforms WD-40 significantly on seized fasteners. It uses a blend of penetrating solvents that wick into rust and corrosion quickly, and it has better staying power than WD-40. It's a solid choice for most shop applications and is more widely available than Kroil.
Best for: Seized bolts, rusted exhaust hardware, corroded fittings
Not ideal for: The most extreme cases of corrosion where Kroil is the better choice
Kroil
Kroil is widely considered the best penetrating oil available for severely seized fasteners. It has an extremely low surface tension that allows it to wick into incredibly tight spaces between corroded threads. Independent tests consistently show Kroil outperforming other penetrants on heavily rusted fasteners. It's the go-to for mechanics dealing with exhaust bolts, rusted suspension components, and anything that's been seized for years.
Best for: Severely seized fasteners, exhaust hardware, long-term corrosion
Not ideal for: Budget-conscious applications where Free All will do the job
Head-to-Head Results
| Product | Penetrating Power | Staying Power | Best Application |
|---|---|---|---|
| WD-40 | Low | Low | Light rust, moisture |
| Free All | High | Medium | Most seized fasteners |
| Kroil | Excellent | High | Severely seized fasteners |
Tips for Getting the Best Results
- Apply and wait — penetrating oil needs time to work. Apply generously, wait at least 15–30 minutes, and ideally let it soak overnight for severely seized fasteners.
- Apply multiple times — reapply every few hours for stubborn fasteners. Each application wicks deeper into the corrosion.
- Use heat in combination — a propane torch applied to the fastener (not the penetrant) expands the metal and helps the oil wick in. Let it cool slightly before attempting removal.
- Impact wrench advantage — the rapid hammering action of an impact wrench breaks the rust bond more effectively than steady torque from a breaker bar alone.
Shop Penetrating Oils at Chem Nut Supply
Chem Nut Supply carries Kroil and Free All penetrating oils in stock and ready to ship. Shop our Penetrants & Lubricants collection for the full selection.