Surface Preparation on Hot-Dipped Galvanized Pipe

" the degree of surface preparation on galvanized pipes in order to apply three coat system as specified in project spec." should be as specified by the coating manufacturer.

Optimal surface preparation on galvanized steel is a light sandblast. Similar to SSPC-SP7 with an angular profile of 0,5 to 1,5 mils. depending on how thick your 3 coat system is.

I would suggest to use instead a 2 coat system using a Polysiloxane ( like Carboxane 2000) as top coat. It will last a lot longer than any polyurethane on the market.

If sandblast cannot be done you can use a water blast to remove all the salts. Then using a primer/sealer (polyamide epoxy) similar to Rustbond at a DFT 1 to 2 mils it will encapsulate the galvanized then you can use another epoxy (barrier coat) and topcoat.