Ask the Expert Question-and-Answer Archive

by Ron Joseph

March, 2003

Painting Galvanized Steel Beams for Salt Water Environment

Q. Is vinyl wash a substitute for sandblasting when coating galvanized dipped beams that will be coated with epoxy and polyurethane topcoat and then installed in salt water areas of the world?

A. The suitability of a vinyl wash primer over galvanizing depends on the smoothness of the galvanized surface and the presence or absence of any corrosion preventive compounds that might have been applied by the vendor of the galvanized steel. Also, the selection of a vinyl wash primer vs. that of abrasive blasting will depend on the aggressiveness of the environment in which the beams will be exposed.

If you do decide to apply the vinyl wash primer you should check with the paint vendor that the coating does not contain too much phosphoric acid. Alternatively, after you have applied the wash primer and allowed it to cure, you might need to wash down the surfaces with clean water to remove any unreacted phosphoric acid. Please follow the paint vendor's instructions in this matter, and bear in mind that the application process differs between steel and galvanized steel.

Abrasive blasting offers the advantage of providing the substrate with "tooth"; in other words an anchor pattern that will enhance adhesion of the primer. If you decide to LIGHTLY abrasive blast the galvanized surfaces, you can apply the epoxy primer without also applying the vinyl wash primer. The abrasive blasting must only roughen the top surface of the zinc coating and not remove the galvanizing. That would defeat the purpose of having the zinc there in the first place.

If you are selling your product to locations across the world, then you should first perform accelerated laboratory tests to confirm your selection of the coating system.

I would not want to send my products out without first knowing that they will withstand the rigors of shipping, storage and exposure in an infinitely wide range of marine environments. If you would like my firm to assist you with this please get back to me.