Build to Print Services
The primary function of paint on aircraft radome is to protect it from the harsh environments associated with temperature extremes, high-speed impact with rain snow and abrasive particles, sunlight and high voltage surface charges of static electricity. If the coating gets damaged in service these destructive forces will attack the radome.
Finish Types Suitable for Radomes Primer and Topcoats
Most aircraft grade primers and topcoats are suitable for use on radomes these include:
- Epoxy finishes
Any paint finish that has a pearlescent or “metal flake” appearance should be avoided, since many of these paints contain very small slivers of aluminum or bronze metal called leafing pigments which can have a significant effect on the radar signal
Important Note: Do not confuse these metallic particles with normal metallic salts and oxides found in most primers and topcoats. These ingredients are dielectrics and will not cause radar interference. It should also be noted that radomes do not have to be painted black to perform satisfactorily.
Anti Static Coatings: These coatings contain a graphite or carbon black particles to make them semi-conductive so that static charges do not build up on the radome. These coatings should be used with discretion since a heavy application can result in attenuation of the radar signal. Also ensure that the coating type being considered is suitable for radome use. Since some anti static coatings are more conductive than others.
Erosion Resistant Anti-Static Finishes: Special purpose protective neoprene (MIL-C-7439) and polyurethane (MIL-C-83231) finishes are recommended for radomes where rain erosion is severe. The US Military specification MIL-R-7705 requires radome rain erosion protection on aircraft that operate at speeds of 250 knots or above. This protection should be applied to the leading portin of the radome.
These coatings are available in two classes Type 1 rain erosion resistant and type II rain erosion resistant with anti-static properties.
How much paint should be applied to a radome? For best results with regular aircraft finishes, apply one good topcoat 4 to 6 mils thick over a 2 mil thick primer base. However when special erosion resistant neoprene or polyurethane coatings are applied the required thickness is 10 to 12mils.
When refinishing a radome, do not paint over existing coatings as the cumulative thickness of the old and new finishes will reduce the radome’s efficiency.
Removal of Paint Coatings
- Preparation of radome for topcoat. The radome will be received with a urethane compatible primer that is easily re-activated or proper adhesion of most topcoat finishes.
- Scuff sand the entire radome surface will 180-220 grit abrasive paper. Sand to ensure that all surfaces to be painted are dull without removing primer.
- Caution some types of lightening diverters can not be painted. Segmented and oxide coated diverters are examples of these. Mask to leave these diverters bare.
- Immediately before painting wipe surface with MEK solvent and let flash off.
- Maintenance of coatings. It is very important to ensure that the radome substrate is protected from the elements especially water.
- Chips in the topcoat should be “touched up” as soon as noticed. Just fill in the chips with a brushed coat of the normal topcoat paint.
- Once the paint damage is touched up, plans should be made to refinish the radome as soon as feasible. This will prevent future water intrusion problems.
Radome Paint Problems: Some of the more common paint problems are excessive thickness, application errors and rain erosion.
Excessive Paint Thickness: Radomes with excessive amounts of filler/primer surfaces and/or multiple coats of paint (anything in excess of 12-15 mils total thickness) will experience loss of efficiency resulting from the excessive thickness which de-tunes the radome. Too thick of a coating can reduce the efficiency of a 90% radome down to 50- 60% When a radome requires refinishing it is essential to remove all old paint.
Application Errors: Composite surfaces are especially susceptible to surface contamination which can result in cratering “fish eyes” or flaking paint. To minimize these problems, thoroughly wipe the sanded radome surface several times with fresh clean solvent such as MEK. Change wiping cloths frequently and do not dip or rinse in the solvent container. To help minimize fish eyes, a small amount of the anti-cratering agent specified by the paint manufacturer may be mixed with the topcoat
Modern Catalyzed Epoxy And Polyurethane Finishes have ingredients that are both age and moisture sensitive. Do no use materials that are outdated or improperly sealed. Radome paint defects can also be caused by temperature, viscosity, application rate, gun pressure and dust contamination. Rather than trying to correct these problems by spraying additional coats which increase thickness, it is preferable to remove the poor finish an start over.
Radome Erosion Resistant Caps. Very thin fabric reinforced neoprene caps are suitable for rain erosion protection on high speed aircraft but are generally black.
Formed Polyurethane Film Erosion Caps with a moisture resistant adhesive backing are also acceptable. These “boots” are clear and allow the colors including stripes to show through.
The use of plastic erosion caps is not recommended for the following reasons:
- Moisture can be trapped between the nose cap and the radome. This condition can set up a strong radar reflection and may result in false radar targets and ground clutter in addition to loss of radar range.
- Radomes are designed with precise laminate thickness and skin separations to optimize performance at the radar’s frequency. The use of plastic caps upsets this feaatures.