For sealing cracks in concrete, bricksasbestos sheets etc :
Clean out crack of any dust or loose material
Fill crack with sealing compound using a knife or trowel
For stopping leak round the heads of nails on roof sheets :
Apply a coat of sealing compound round the nail heads with a stiff brush
For repairing holes on roof sheet :
Apply a coating of sealing compound round the hole
Cover the hole with a piece of cloth
Paint over the cloth with sealing compound