I have fielded a few questions lately from pilots regarding composite panels on their aircraft.
On modern business aircraft you can see bubbled paint on composite panels that looks like corrosion. How is that possible on a composite panel?
Some aircraft OEMs use a top layer called ALS on composite panels to dissipate static and provide lightning strike protection. This layer is made of metal mesh of varying types and weights. At some point over the life of the aircraft, a checkerboard pattern of bubbled paint could form on the panel. That indicates the ALS layer on that panel is forming some corrosion.
This is not a particularly difficult fix when caught early. If left unaddressed, it could become costly due to the amount of surface area that will need work if the corrosion is allowed to spread.
The solution to removing corrosion on a composite panel is to sand down the paint and ALS layer in the corroded area, then reapply a new ALS layer a little larger than the sanded area. If the area is small, you may not need to do a repair, only deal with the corrosion, fill the void, and reapply paint.
If the corrosion is allowed to grow unchecked, the repair could very well take up an entire panel and become extremely costly. The sooner it is taken care of, the easier it is to repair and costs are more manageable.
The best time to take care of this, if possible, is when your aircraft is down for paint.
Cessna CIL-21-03: Water Separator Corrosion Repair