What the coating does, and what it doesn't
Oscar Liang describes conformal coating as a thin protective film that follows the contours of the board and insulates components and solder joints, so moisture can't bridge them. The goal is water resistance, not waterproofing. He recommends it even if you never plan to fly in the rain, because dew and wet leaves after a landing or crash are enough to short an exposed joint.
The limits matter as much as the benefits:
- Plugs, sockets and buttons stay exposed. Oscar says it plainly: the coating does not make the quad fully waterproof.
- The libdrone project's reference page on coating adds that it does not seal connectors or openings in components, and does not prevent damage if the drone ends up underwater.
- KiwiQuads calls it extra protection against shorts when a drop finds its way onto the board, not invincibility against rain or snow.
Coating raises your odds. It doesn't turn a soaked quad into one you can keep flying.
Silicone or acrylic
The two common types are silicone and acrylic. Acrylic is easier to remove, because a soldering iron burns through it quickly. Oscar's objection is heat: video transmitters and other parts in a quad can get hot enough to soften an acrylic coat. That is why he prefers silicone for drones. It takes more heat to melt, yet a soldering iron still gets through it.
The product Oscar recommends, and the one KiwiQuads uses, comes from MG Chemicals. Get the version with a UV indicator, so the coating glows under UV light and you can see where it is missing. MG's technical data sheet describes 422C as an acrylic-silicone blend with a service temperature range of -40 to 200 °C. According to Oscar, a 55 ml bottle is enough for 50-100 quads. It comes with a nail-polish style brush in the cap.
Use the brush, not the spray. Oscar's reason: spray gets onto connectors and buttons you don't want to coat, and it is hard to cover evenly without precision. KiwiQuads also works with a brush.
Build first, coat later
Coat a quad that already flies. Oscar suggests finishing the build, test-flying it to make sure everything works, and only then coating it, because coated pads are harder to solder later. KiwiQuads says the same: the best moment is straight after the build, once you have confirmed the joints are good.
You can still solder through it. MG says 422C can be soldered through for repair. KiwiQuads notes that the coating goes gummy around the joint and you need to wipe the tip more often, otherwise it is no different. Whichever source you follow, the rule is the same: after you resolder a joint, coat that spot again.
On an older quad, Oscar removes tape and heat shrink from the boards first, then cleans them with an old toothbrush and isopropyl alcohol. MG's data sheet also recommends cleaning with isopropyl alcohol so the surface is free of oils and dust. libdrone points to one more reason to coat a clean board: if a board has already been wet, conductive residue may be left on it, and coating over it seals that residue in place.
What to coat and what to leave bare
Coat the exposed metal: chip pins, solder pads, solder joints and anything that could conduct when wet. KiwiQuads names the flight controller, ESC and VTX as the obvious candidates, since they have no moving parts.
Keep the coating off these, from Oscar's list:
| Part | Leave uncoated |
|---|---|
| Flight controller | Boot button, USB port, barometer |
| FPV camera | Image sensor |
| VTX | Channel button |
| Receiver | Bind button |
The barometer gets a warning from every source. libdrone explains why: coating over the sensor or its vent can block the pressure port and throw off altitude readings. KiwiQuads adds switches and sockets to the list, because coating can seize them or stop a plug from making contact. Two tips make that easier. Oscar plugs in every connector before coating so the coating can't get inside the contacts. libdrone masks connectors and the barometer with Kapton tape.
Motors don't need coating. KiwiQuads explains that the windings are enamelled, which already stops them shorting. Coating inside a motor is more likely to gum it up and may seize the bearings.
Some flight controllers and ESCs come pre-coated from the factory. Oscar warns that factory spray coating can be uneven and miss spots, so check it under UV light before you trust it. libdrone, on the other hand, does not coat a VTX that already has a manufacturer coating, because a second layer can trap heat. When a VTX already has a factory coat, touch up the gaps you find rather than putting a second full layer over it.
Applying it
- Work in a well-ventilated room. Oscar warns the coating can be carcinogenic, so don't breathe the fumes. KiwiQuads suggests reading the safety data sheet first.
- Brush it on like nail polish: thin, even and shiny. MG gives a recommended film thickness of 25-75 µm, so think thin layer, not a pool. libdrone warns that bubbles and thick pools can crack under vibration.
- Coat both sides of the board. In the comments under his guide, Oscar answers that ideally yes, you should.
- For small gaps, for example between capacitors and resistors, Oscar blows the coating in with compressed air, held far enough away not to make a mess.
- Check the coverage under a UV torch. The coated areas should glow evenly. KiwiQuads notes that a blue LED torch, or even sunlight, helps if you don't have a UV one. Touch up any gaps.
- One or two coats are usually enough, according to Oscar.
- Clean the rim of the bottle before closing it. Dry silicone glues the cap shut.
Drying time
The sources give different numbers, and the difference is between dry to the touch and fully cured:
- MG's data sheet for 422C: dry to handle in 10 minutes after one coat and 15 minutes after two, with a minimum of 2 minutes between coats. A full cure takes 24 hours at room temperature, or, after 10 minutes of air drying, 20 minutes at 65 °C or 10 minutes at 80 °C in an oven.
- KiwiQuads: expect 30-40 minutes for the coating to dry.
- Oscar: at least 30 minutes, but leave it overnight to be safe.
If you're not in a hurry, let it cure overnight. That matches both Oscar's advice and MG's 24 hours at room temperature.
Removing it
Oscar removes silicone coating with isopropyl alcohol and a cotton bud. A few gentle rubs take off a thin layer. To strip a whole board, he soaks it in alcohol, scrubs it with a soft toothbrush and lets it dry completely. A hot soldering iron also burns through it. MG mentions removal with appropriate strippers. libdrone is more cautious and treats coating as practically permanent, because solvents can damage traces and the solder mask. Treat removal as possible but not risk-free, and aim to keep the coating off pads you will solder later.
Flying wet and crashing into water
Coating is only one part of flying in the rain. Oscar lists the rest: water on the camera lens and goggles that fog up, less thrust from wet props so you need more throttle, a weaker video and control signal, and gusty wind. After the flight, dry the quad straight away, because leftover water causes corrosion over time.
If the drone ends up in water, Oscar's procedure is:
- Get it out as quickly as possible and unplug the battery immediately.
- Rinse the electronics in distilled water to wash out minerals. Brushing them with isopropyl alcohol is even better. Do this on the electronics, not the motors.
- Let it dry completely. Skip the rice. Put it in front of a fan for at least a day.
Salt water is a different story. In his write-up about crashing into the sea, Oscar says that fresh water is usually fine with a rinse and drying, but with sea water you should be ready to lose the quad, or at least some of its parts. His own quad was nearly dead, apart from the enclosed camera, motors, frame and a few small parts. Corrosion doesn't stop once it starts, so a part that seems fine today can degrade or fail within a few days. Before powering a drowned quad again, he checks for shorts with a multimeter, uses a smoke stopper if possible, and range-tests the receiver and VTX carefully.
One last thing about warranty. A commenter under Oscar's guide reports that two resellers refused ESC warranty claims because they treated the coating as a modification. Check the shop's terms before you coat a new part.