If you're in need of lap sealant it doesn't get much better then Firestone's Lap Sealant.
Firestone Lap Sealant is designed to seal and mechanically protect the exposed cut edge of QuickSeam Products. Lap Sealant is also used as a general purpose sealant in other applications as specified by current Firestone specifications and details.
The sealant is applied with a mastic gun at the specified location using the plastic nozzle applicator supplied by Firestone, being sure to keep the nozzle applicator centered over the lap step-off. Take care to leave a mound of Lap Sealant directly over the splice edge.
Flammable - Keep away from sources of ignition. Do not smoke when using. For professional use only. Use only in well-ventilated areas. Avoid contact with skin and eyes. Refer to Material Safety Data Sheets. Keep out of the reach of children. Recommended cleaner is rubbing alcohol followed by soap and water.
Solvents: Light aliphatic solvent
Solids (%): Min. 80
Specific Gravity: 1.34 - 1.46
Flash Point: 11
View Firestone Lap Sealant Application Video