Applying conformal coating properly makes a huge difference in protecting your electronics from the elements during racing. I've saved countless flight controllers and ESCs from crashes in wet grass and dusty tracks by doing this right.
Start with preparation, which honestly matters more than the application itself. Remove your flight controller and ESC from the frame. Use 99% isopropyl alcohol and a soft brush to scrub away any flux residue, fingerprints, or dirt. Let everything dry completely for at least 30 minutes. Any contamination under the coating will trap moisture instead of repelling it.
Next, mask off anything that shouldn't be coated. Use Kapton tape or regular masking tape to cover barometric pressure sensors (the small silver squares on many flight controllers), buzzers, LEDs if you want them bright, and all connectors including USB ports, battery plugs, and motor pads. Also mask the gyro chip if your manufacturer recommends it, though most modern chips handle coating fine. I learned this the hard way when I coated over a baro sensor and my altitude hold went haywire.
For application, you have two main options. Brushing gives you more control and uses less material. Dip a small brush into your coating (I use MG Chemicals 422B or Corrosion-X HD) and apply thin, even strokes in one direction. Don't overload the brush or you'll get runs and drips that are nearly impossible to fix. Build up two or three thin coats rather than one thick coat. Each layer should look slightly glossy but not pooled anywhere.
Spray application is faster for multiple boards. Hold the can about 20 centimeters away and use smooth, sweeping passes. Again, multiple light coats beat one heavy coat. The first coat might look patchy, but subsequent layers fill it in.
Drying time varies by product. Most coatings feel dry to touch in 30 minutes but need 24 hours to fully cure before flying. I wait 48 hours to be safe, especially in humid conditions.
Don't coat the entire ESC if it has exposed MOSFETs that need cooling. Just coat the control circuitry side. For 4-in-1 ESCs, you can coat everything since they're usually enclosed anyway.
One trick I picked up from racing in the Pacific Northwest: after coating, I still add a small heatshrink or electrical tape cover over the USB port. Connectors are the weak point even with good masking, and this extra barrier has saved me during surprise rain races.
