I'm not ultra familiar with foam roofs, although I do install a fair bit of elastomeric roofing products. What I do know about foam roofs is that the elastomeric IS the waterproofing, not the foam. Therefore it is imperative that what ever be installed be installed absolutely water proof.
HOWEVER I have some questions for you. What type of elastomeric is to be installed? Acrylic, Urethane, SEBS, Silicon, Urea? I ask this because each will have their own application rates and methods. For example, a polyurea would not need two coats for sure.
I also ask how many gallons per square would be applied and to what dy mill thickness would the product be applied?
My suggestion would be to contact the manufacturer directly. Having said that I took the liberty to check into ER Systems and read their specification for you. http://ersystems.com/uploaded_files/..._DG0032-D_.pdf
3.03 COATING APPLICATION
A. Base Coat
1. Base Coat of the selected chemistry (urethane, acrylic or silicone) shall be applied to foam repair areas the same day the
foam is applied.
2. Sufficient Base Coating must be applied to the repaired areas to provide a smooth transition to the original coating.
Application is typically 32 wet mils minimum and tied into the original coating for 3-4" beyond the repair.
3. Primer shall be applied to the original coating if the adhesion test dictates the need.
4. Base Coat (urethane, acrylic or silicone) is then applied to the entire roof at a minimum of 16 wet mils to provide a
minimum of 10 dry mils of protection at all points.
Keep in mind that any other manufacturer may have a different specification, so this specification is meaningless unless the product to be used is ER Systems. This is why you shoudl contact the manufacturer in question and ask them yourself.
You may learn a little more about roof coatings at my website, although there is no information about spray foam... Roof Coatings