Ask the Expert Question-and-Answer Archive

by Ron Joseph

December, 2007

Painting an Aluminum Beam on a Catamaran

Q: I have been sold a two component primer made by Berger:

• Etching primer base
• Etching primer converter

The salesperson said: just mix it and apply...

The application is: an aluminium beam on a catamaran. The beam is coated 1994 by the fabricator with an unknown primer and white finish. Over the years some flaking took place and some oxidation at attachment points.

I realize that a proper job would be to dismount everything and clean to bare metal before refinishing. However this is too much work.

For a quick fix, can I indeed use the primer in question and paint over existing coating? I am somewhat weary about applying the "etching primer" without knowing what it will do exactly.

Thank you for your help

A: Your concern for use of the etching primer is well founded. The etching primer, more commonly called a "wash primer" is intended solely for use in direct contact with the aluminum. You should not apply it over old paint because it contains phosphoric acid.  Unless the acid can react with the metal substrate you will simply have surplus acid sitting on the surface, and it will react with whatever topcoat you intend to apply. Ultimately, you can expect to get blisters and poor paint adhesion.

Unless you get down to the bare metal surface, you can scuff sand the old coating and then apply an epoxy primer followed by a marine top coat, such as a polyurethane.

Please read all the instructions on the label of the cans, and please take all safety precautions seriously.

Regards,

Ron Joseph