I’ve been spraying Fluid Film & Woolwax for ten years, and have applied it for dozens of clients in my shop.
Prepare the Vehicle
At minimum, removing any flaking or deep/thick rust is necessary to success. Flaked or layered rust will get the film on the top surface, but prevent it from getting into the layers underneath, leaving them susceptible to continued exposure to air/water and corrosion. You don’t necessarily need to remove or do much of anything to prep single-layer surface rust. This is one of the benefits of a wet film lanolin oil coating like this is — it doesn’t require a full removal of surface rust to be effective (vs something like a paint or other drying coating) — the oil soaks into the rust and stops or slows it by isolating it from air and water.
If you’ve got a significant amount of dirt, sand, salt, water, etc on the underside of the vehicle, or just want to make sure it’s clean, give it a good flush before applying the lanolin coating. A carwash with underbody flush should do fine for most street vehicles, but if you’ve had it off-road or have years worth of dirt, sand, whatever underneath, you might need to get more involved. You can run an oscillating yard sprinkler under the vehicle for a while to give it a good flush. Or get under there and clean it out more hands-on, manually. Then give it time to dry out – or help speed the drying with a fan.
One other prep tip if you want to be extra thorough — with the ultimate goal in mind of preventing our cars from rusting out — look into places up under your vehicle where dirt, sand, or debris could accumulate. These are common hotspots for the kind of rust holes that kill cars because 1. sand or debris will retain moisture to accelerate rust, and 2. they’re usually collected there in the first place due to having no exit for them get out, which means water will also sit there longer to accelerate rust. One example is in front of your A-pillar/front doors, and behind your front tires, down at the bottom of the front fender there’s commonly a pocket of space behind the wheel-well liner where road debris can accumulate, or leaves/sticks/etc can run down from your cowl intake drain area. If you’re spraying water on your car with a hose and you see some debris or dirt flush out from a crack or wherever, try to look into where that was trapped and there’s probably a lot more where it came from. Another example is any kind of body, frame, or step-side mount or bracket where there’s a cup-shaped recess, but no weep hole for water or debris to flush out. You can feel around with your fingers for spots like this (wear gloves for protection if you want), and you’ll commonly feel sand, rocks, or other debris collected there. These are the kinds of spots I often find when it’s too late and the rust has made a hole and like a gallon of debris or sand dumps out.
Apply the Coating
1. Use PPE. A respirator can help protect against getting rust, or especially-harmful brake dust in your lungs during prep. Also consider a respirator while spraying lanolin wet-film coatings. They are generally organic materials and non-toxic, so in my estimation not bad if you’re outdoors or in an open garage. But the fumes can accumulate in the air if you’re working for some hours in a closed garage, for example. Eye protection is a must. With the air pressure and spraying there’s often dirt or bits flying that could get in your eyes. I wear rubber gloves, long sleeves, and a hat; more to keep off the grease and dirt from under the car than the Woolwax, which is pretty clean and easy to wipe off.
2. Definitely avoid getting the lanolin coating on brake discs, or inside brake drums. You can put a bag or something on to mask these parts, or if you’re thoroughly careful you can just avoid overspraying onto them.
3. The only other thing I’m aware of which can be negatively affected by lanolin oil is some soft rubber parts or weatherstripping can get a little expanded if they’re soaked with the oil. I’ve sprayed it on every incidental rubber piece under the vehicle, bushings, suspension parts, etc. for years with no noticeable adverse effects. But I have seen others who sprayed in the door jamb on the weatherstripping and it showed a little bit wavyness (word?) due to some swelling expansion. See RepairGeek videos on Woolwax/Fluid Film on Youtube for great extensive reporting on lanolin wet-film coating results over the years. They document somewhere the effects on rubber, and overall cover just about everything you’d want to know about lanolin wet-film coatings.
4. Generally avoid getting the lanolin coating on exhaust parts, though it’s not going to hurt them. It’ll just smell a bit until it’s burnt off. I don’t mask exhaust parts, just avoid directly spraying them. But I err on the side of getting coverage on everything else around them even if a little overspray is going to get on the exhaust. Usually the little bit of overspray burn-off and general Woolwax smell is gone after the first drive at full engine temp, or after a few days.
5. My baseline standard application is basically everything the spray gun can ‘see’ from the underside of the vehicle, plus what I can access with the 360 spray nozzle on the end of a tube/wand: inside frame rails for frame vehicles, top-sides of suspension and other parts under the vehicle, and anywhere I can spray up under/behind body panels/pinch welds/fenders. You can also get into additional thorough coverage inside less accessible parts of body panels by removing headlights/taillights, panels, plugs, etc., or even drilling new holes in places, spraying inside, and then plugging the holes with rubber plugs (but those places are naturally more isolated from moisture in the first place, unless they’re known problem ingress/rust hotspots).
6. Regular reapplication, especially for salt-belt vehicles, is necessary to keep up the full protective coverage of the coating. Woolwax/Fluid Film has a recommended annual reapplication schedule, Surface Shield every two years (I don’t have experience with that 2-year coverage claim). I have tested this by leaving more time between coatings. Specifically with Woolwax, after 1.5 years spanning two winters, some wash-off areas were definitely starting to show surface rust reappearing (see header photo from this test – this is after about 5 years of at least annual coating and then 1.5 years without). Where it’s coated/soaked with oil it just looks black and oily, no visible rust nor rust progression. Where it washes off — primarily at prominent points around suspension parts where tires spray water/slush/salt — you can see the spots that wash off, dry out, expose the surface rust, and then turn orange rust-colored again. After 5+ years of annual recoating, probably 90% of the surface area of the underbody and frame was still completely coated after the 1.5-year span, but those high wash-off areas still need re-coating. And it doesn’t hurt to just touch up the whole thing as the whole underbody benefits from additive reapplication, and decreases chances that you missed coverage in some little nook up inside a frame rail or somewhere you can’t see.
Materials
I’ve used Fluid Film and Woolwax, and they’ve worked great for me, but there are others out there like Surface Shield. Surface Shield has been reported to hold up longer than Woolwax (it has a 2-year recoat schedule, maybe has a wax component to it, and may creep a little more/thinner), but I don’t have personal experience with testing that yet. It used to cost about double per gallon vs Woolwax, but I think prices are closer now – I’ll update if/when I get to test Surface Shield. Maybe I’ll start with that one on my next vehicle for a comparison test.
Kellsport’s Pro Gun Kit is my recommendation for the best tool set to spray Woolwax/Fluid Film. But I started out using Fluid Film aerosol cans to coat my cars before I had a garage space and air compressor to do it. I’d get a dozen cans to cover a couple years worth of coatings. Note, you’ll use more than average if you’re coating a vehicle for the first time, and may need more frequent re-coating if you’re off-roading and touching ground. I regularly run my vehicles through a carwash with underbody flush, especially and more frequently in winter (every couple weeks, or whenever temps get above freezing), and the Woolwax holds up fine between annual re-coatings.