Solar Collector Paint

Thurmalox 250 Selective Black Solar Collector Coating

Solar CoatingDescription

Thurmalox 250 is a selective black silicone-based heat resistant coating designed for use on metal surfaces of solar collector panels. It selectively absorbs solar wave lengths with the greatest heat content (visible and infrared wave lengths) and therefore collects heat energy more efficiently than ordinary nonselective black paints which emit (reflect) a significant amount of incident solar radiation.

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Thurmalox 250 will function with maximum efficiency when used on collector panels having glazed covers. It is unaffected by high heat and humidity and exhibits no outgassing during stagnation periods. See Application and Technical Data sections for additional information.
In cases where a non-selective coating is required, Thurmalox 270 Black coating is recommended. See Bulletin 270 for details.
For unglazed collectors such as outside surfaces of passive houses and Trombe walls, use Endcor 902 Black Latex passive solar coating. See Bulletin 902 for details.


  • Selective surface
  • Resists outgassing to 400°F (204°C)
  • Heat resistant to 1,000°F (538°C)
  • Doesn't peel, flake or chalk
  • Won't bleach or discolor
  • Withstands repeated thermal cycling
  • Resistant to UV degradation
  • Dries for handling in 30 minutes
  • Air dry or heat cure
  • Long lasting
  • Available in 13 oz. (net wt.) aerosol cans

Surface Preparation

Aluminum: Prepare surface with Endcor 450 Rust Rinse (see Bulletin 450). Wash thoroughly with water and dry. Apply Thurmalox 250 as noted below.
Stainless Steel and Copper: Remove oils and grease by wiping down surface with Dampney 170 Clean-up Thinner.


Redisperse any settled-out pigments by stirring with a paint paddle followed by thorough mixing to a uniform consistency with an explosion-proof or air-driven power mixer. Do not open containers until ready to use. Keep lid on container when not in use.

Thinning Required

Do not apply Thurmalox 250 without thinning. The choice of thinner will influence the coating's average selective characteristics. For optimum selectivity, a slow-evaporating thinner such as Dampney 100 should be used. For production line application where a faster dry time is required, use Dampney 110 Thinner. The fastest thinner available is Dampney 140. NOTE: Fast-evaporating thinners such as Dampney 110 and 140 may promote dry spray, which could result in thicker than recommended films. To ensure the optimum combination of application speed and selective properties, consult your nearest Dampney representative for specific recommendations.


For best results, apply Thurmalox 250 by spray. IMPORTANT: To ensure maximum performance, coating film thickness must be carefully controlled. For maximum selectivity, spray apply a very thin coating, only enough to just hide the metal surface and to make it uniformly black in appearance. Apply only one coat to a maximum dry film thickness of 0.10 mils (0.4 mils wet film thickness - unreduced; 0.8 mils wet film thickness - reduced 1:1 with the recommended Thurmalox thinner. In the case of steel surfaces additional consideration must be given to the potential problems of corrosion and rusting of collector surfaces and appurtenances. Under such circumstances corrosion can be effectively prevented by applying a prime coat of thurmalox 245 Silicone-Zinc Primer prior to application of the Thurmalox 250 Selective Black Coating. The use of thurmalox 245, however, will inhibit the selectivity of the coating system. Apply one coat of Thurmalox 245 to a dry film thickness of 1-1.5 mils. Apply one coat of Thurmalox 250 to a dry film thickness of 1-1.5 mils. Total system dry film thickness: 2-3 mils. See Bulletin 245 for detailed information.

NOTE: To ensure maximum performance characteristics, emissivity measurements should be taken as a control procedure. See Selectivity Data in Technical Data section.


Best results are achieved with conventional spray application or aerosol spray. Reduce with equal volume of Dampney 100 Thinner. Adjustments for proper spray patterns may be required.

As a guide only...
Spray gun  -  DeVilbiss P-MBC or JGA
Air hose  -  5/16" (I.D.)
*Fluid hose  -  3/8" (I.D.)
Air cap  -  704
Fluid nozzle  -  FX (0.042")
Pot pressure  -  2-5 lbs.
Atomizing pressure  -  30-35 lbs.
Needle adjustment  -  Half open
Distance from work  -  8-19"
*Smaller hose diameters, or hose lengths greater than 25 feet may make it necessary to increase pressure.

Drying Time

Thurmalox 250 dries to handle in 30 minutes. For optimum service, coating can be baked one hour at 350-400°F although curing will occur in service.


Clean all equipment with Dampney 10 Clean-up, or Thinner, or xylol.


Store Thurmalox 250 in a cool place, preferably below 80°F.

Characteristics Thurmalox 250
Color Black
Operating temperature range Ambient to 500°F (260°C)
Surface temperature at time of application 50-120°F (10-49°C) max. (spray cans, 75°F (24°C) max.
Maximum temperature resistance 1000°F (538°C)
Percent (%) Solids by volume 20
Weight per gallon 9.24 lbs. (1.1 kg/liter)
Theoretical coverage per gallon* 320 mil. sq. ft. (7.9 sq. m. / liter)
Selectivity Data 50°F-500°F (10°C-260°C)
Absorptivity (alpha)-reduced 1:1 with
Thurmalox 295 Thinner
0.5 mils wet (.06 mils dry) [1.5 microns dry] 0.96
1.0 mils wet (.13 mils dry) [3.3 microns dry] 0.96
2.0 mils wet (.25 mils dry) [6.4 microns dry] 0.96
180°F (83°C) Emissivity (epsilon) – reduced
1:1 with Dampney 100 Thinner
0.5 mils wet (.06 mils dry) [1.5 microns dry] 0.52
1.0 mils wet (.13 mils dry) [3.3 microns dry] 0.65
2.0 mils wet (.25 mils dry) [6.4 microns dry] 0.80
180°F (83°C) Emissivity (epsilon) – unreduced
2.0 mils wet (.50 mils dry) [12.7 microns dry] 0.90
Flash point (Pensky-Martens)
Thurmalox 250 81°F (27°C)
Thurmalox 295 Thinner 81°F (27°C)
Drying time at 77°F
Set to touch 10-15 minutes
Heat cure at 350°-400°F 1 hour
Shelf life 1 year