http://www.liquidroof.net/elastomeric_coatings.html

Like any paint application, preparation is key to a long lasting surface coating. Pressure wash the present surface, and remove/sand any loose paint. Any rusty areas should be treated with a rust converter.

The elastomeric company you choose will recommend a brand of primer that is compatable with their products. It is important to use the approved products for the best final adhesion. The elastomeric coating can be rolled or brushed, which is the easiest application for smaller areas. Commercial buildings are often sprayed with high pressure guns.

You will have an open window time period, when additional coats can be added before the previous coat fully cures. Follow the application instructions of the manufacturer and you shouldn't have any difficulties.