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Putty will dry, even if top coated with a sealant, in time. This will depend on how much of the surface is exposed to the air and how porous the surrounding material is. Dry putty shrinks, becomes brittle and cracks. Mastic retains it's elasticity whether coated or not and almost regardless of the nature of the material it is used to seal or fill, although it will dry eventually, it will be a very long time.Putty is also unable to deal with expansion and contraction very well and is highly susceptible to any movement. Mastic, because it does not 'set', is able to move with any such movement.