Automotive · Head-to-head
Woolwax Auto/Truck Undercoating (1 Gallon) vs Fluid Film Black Aerosol and Non-Aerosol
Which should you buy? Our verdict on both, pulled from our full The Best Car Undercoating: 4 Compared (and One to Skip) comparison.
Woolwax is the thickest of the soft coatings here and the only one whose maker publishes a multi year interval and a specific respray procedure over the old coat. Fluid Film Black does the same job in a thinner, more forgiving form that is far easier to apply the first time.
Woolwax Auto/Truck Undercoating (1 Gallon)
Buy itA thick, solvent free wool grease coating that never dries and that you respray straight over the old coat.
- Kellsport states it does not dry out, stays soft and pliable, and keeps creeping into inaccessible areas.
- Stated protection of two to three years, with the exposed underbody re-treated every other year.
- Reapplication needs only a hose rinse, not stripping, because you spray fresh material over the existing coat.
- The label tells you to warm the container to 90 to 100 F, so a cold garage is the wrong place to work.
- Needs a low pressure undercoating gun at 60 to 90 psi, which is an extra purchase over an aerosol.
- Stays greasy, so road grime clings to it and anything you brush against underneath will mark.
Fluid Film Black Aerosol and Non-Aerosol
Buy itThe thinner lanolin coating that creeps furthest into seams and is by far the easiest first undercoating job.
- Eureka Chemical states it remains soft and pliable, which makes it self healing and impossible to crack.
- Sold as an aerosol as well as in bulk, so you can start small.
- Maker states it is non-conductive and will not harm electrical connections, and has no adverse effect on plastics.
- Stated protection is up to a full year, shorter than the multi year figure Woolwax publishes.
- Thinner film means more of it washes off exposed areas hit directly by road spray.
- Eureka flags that it may soften some vehicle undercoatings, including undercoating type paints.
Side by side
The 8 criteria below are the ones our full comparison judged every product on, narrowed to these two.
| Woolwax Auto/Truck Undercoating (1 Gallon) Our pick | Fluid Film Black Aerosol and Non-Aerosol | |
|---|---|---|
| Our verdict | Buy | Buy |
| Best for | Salt belt owners who will commit to a recurring spray job and want the longest interval between them. | Anyone doing this for the first time, or covering seams and cavities where creep matters more than film thickness. |
| Price tier | $$ · mid-range | $ · budget |
| Base chemistry | Raw wool grease (lanolin), solvent free | Processed wool wax plus refined petroleum oils |
| Film after application | Does not dry, stays soft and pliable | Will not dry out, remains soft and self healing |
| Maker's stated interval | Typically 2 years; exposed underbody every other year | Up to a full year per application |
| Applying over existing rust | Label: can be applied over existing surface rust; remove flaking or bubbling rust | Not formulated to remove rust; maker says it stops existing rust spreading |
| Application temperature | Warm product to 90 to 100 F before spraying | Not published |
| Rubber, plastics, wiring | Will not harm paint; safe on plastics and oil resistant rubber; may swell non oil resistant rubber | No adverse effect on plastics; non-conductive; may swell non oil resistant rubber |
| How you apply it | Low pressure undercoating gun, 60 to 90 psi | Aerosol, or bulk with a gun |
| Published corrosion test data | None published | Cites MIL-C-16173 Grade 2 soft film salt spray requirement |
Which one is right for you?
Pick the Woolwax Auto/Truck Undercoating (1 Gallon) if
salt belt owners who will commit to a recurring spray job and want the longest interval between them.
Know before buying: the label tells you to warm the container to 90 to 100 F, so a cold garage is the wrong place to work.
Pick the Fluid Film Black Aerosol and Non-Aerosol if
anyone doing this for the first time, or covering seams and cavities where creep matters more than film thickness.
Know before buying: stated protection is up to a full year, shorter than the multi year figure Woolwax publishes.
The bottom line
Our overall pick is the Woolwax Auto/Truck Undercoating (1 Gallon) - A thick, solvent free wool grease coating that never dries and that you respray straight over the old coat. Choose the Fluid Film Black Aerosol and Non-Aerosol instead if anyone doing this for the first time, or covering seams and cavities where creep matters more than film thickness.
Want the full picture, including the other options we compared and the popular one we'd skip? Read the complete The Best Car Undercoating: 4 Compared (and One to Skip).