Roskote R28 Rubberized Mastic by Royston

Fast-setting, High Performance Coating with Outstanding Resistance to Cathodic Disbondment

Royston Roskote R28 Rubberized Mastic

Royston Roskote R28 Rubberized Mastic is a cold-applied coating with high electrical resistivity designed to protect underground steel pipes, tanks and other structures against corrosion.

Royston Roskote R28 Rubberized Mastic bonds firmly to clean steel surfaces without use of a primer and also to coal tar and hot applied asphalt. It forms a thick, tough coating which will not oxidize, craze, alligator, crack, check, loosen or scale. It provides a high degree of electrical resistivity and shows excellent performance in cathodic protection or stray current situations. It withstands soil stresses and resists attack by alkalis, acids, salts and moisture.

Royston Roskote R28 Rubberized Mastic should be used as a general purpose coating for underground protection of all fabricated steel products, such as pipes, conduits, valves, fittings, tanks, hydraulic cylinder castings and structural steel. For field coating joints of mill coated pipe, it can be used in conjunction with reinforcing fabric such as Royston Glas-Wrap.

In certain areas in or near petroleum refineries, gasoline bulk plants, service stations, oil producing fields and similar locations, Roskote A-51 Plus which is resistant to both oil and gasoline should be used.

How to Use It

  • Handling Precautions: Royston Roskote R28 Rubberized Mastic is a solvent based material which should be kept away from heat or open flame and should be used only with adequate ventilation. Repeated or prolonged contact with the skin should be avoided.
  • Surface Preparation: Surfaces should be dry for maximum adhesion. Remove rust, scale, soil, mud, oil, grease and other contaminants in the field by wire brushing. Sandblasting or chemical treatment may be more convenient for in-plant surface preparation. No adhesive is required except on newly galvanized surfaces which should be coated with a suitable phosphate wash primer prior to mastic application.
  • Stirring: Royston Roskote R28 Rubberized Mastic should be stirred thoroughly before using.
  • Thinning: Thinning is not required except for cold spray at temperatures below 60°F. If excess thickening occurs because of solvent evaporation from opened containers, original viscosity may be restored by adding Royston Thinner 11 (Toluene).
  • Application: Royston Roskote R28 Rubberized Mastic is applied cold by brush, spray, flow coat, dip or rubber glove. Apply two coats of Royston Roskote R28 Rubberized Mastic at the proper thickness, allowing the first coat to dry to the touch before applying the second.

Typical Properties


A pyrobituminous resin processed by homogenization with inhibitive pigments and aromatic solvents.



Volume Resistivity

2.12 x 1013 ohm cm3

Application Temperature

32°F to 120°F

Service Temperature

0 to 250°F

Shelf Life

At least one year

Weight per Gallon

9.42 lbs

Specific Gravity


Percent Solids:
   By Volume
   By Weight

69.5% ±2%

Flash Point

44°F Seta Closed Cup



  • When brushing Royston Roskote R28 Rubberized Mastic, it should be applied liberally with a generous amount on the brush at all times. It should never be brushed thin.
  • For spray application, booster type spray equipment is required because of Royston Roskote R28 Rubberized Mastic's heavy consistency. At temperatures above 60°F, Royston Roskote R28 Rubberized Mastic may be sprayed as received without thinning or heating, but below 60oF it is usually necessary to thin with one quart of thinner to each five gallons of Royston Roskote R28 Rubberized Mastic, or to use an in-line heater set at 120°F maximum. Either air types of airless spray equipment commercially available from several manufacturers may be used. The following equipment and procedures have provided satisfactory results, but alternates may be equally successful.
  • For Use With Reinforcing Wrap. Using with a reinforcing woven mesh such as Royston Glas-Wrap, the first step is to apply one coat of Royston Roskote R28 Rubberized Mastic and allow to dry to the touch. This amount of drying is necessary before proceeding further to prevent the wrap from pulling through the coating to the metal surface. From this point there are two alternate procedures in common use.
  1. Apply the wrap and then put on a second coat of Royston Roskote R28 Rubberized Mastic.
  2. Apply the second coat of Royston Roskote R28 Rubberized Mastic and then wrap use either while the Royston Roskote R28 Rubberized Mastic is still wet or after it has dried to the touch. Wrap tightly to embed the mesh in the coating.
  • Drying Time: Royston Roskote R28 Rubberized Mastic usually dries to the touch in approximately 20 minutes. In 1-1/2 hours, it is sufficiently dry for light handling and backfilling. If it is necessary to backfill while the Royston Roskote R28 Rubberized Mastic is still wet or soft, apply pipeline felt to protect against damage to the coating.
  • Coverage: Each coat of Royston Roskote R28 Rubberized Mastic should be applied at a coverage rate of 75-80 square feet per gallon, which will provide a dry film thickness of about 12 mils.
  • Holiday Detection: The suggested setting for holiday detector instruments is 125 volts for each mil of dry film thickness. The coating must be thoroughly dry before testing. Allow at least 24 hours for complete solvent evaporation.
Air Type Spray


Graco 8:1 ratio minimum or equivalent.
President or Bulldog 10:1 ratio recommended.


3/8" to 1/2" I.D.
3/4" to 1" I. D.


Heavy material gun

Spray Tips

3/16" minimum. External Win Air Cap


Will vary wit temperature and pump output ratio, 50-90 psi material pressure is normal. Atomization pressure 40-80 lbs.

Airless Spray


Graco Bulldog or equivalent. Pump must be capable of an output of three gallons per minute.


Material hose 1 /2" minimum, for lengths over 25 feet use 3/4" ID. Use high pressure, wire braid hose.


Use mastic type airless spray gun, Graco Hydra mastic Gun or equivalent.

Spray Tip

.031" maximum