Porous Primer?

[John N. Packard]

The current issue of Antique Automobile has an interesting article by Red Burke on the restoration of a 1946 Hudson convertible. He makes the statement on page 54 "that primer is porous and absorbs moisture like a sponge." Can anyone clarify this for me? I have always assumed, perhaps incorrectly, that a coat of primer would protect the metal from rusting.<P>jnp

[Guest]

He is quite correct if refering to older laquer-based primers and primer surfacers. These paints were pigment heavy and not meant to seal against moisture. Over time they will absorb considerable moisture. However, if using a modern urethane-based primer/surfacer, these primers seal the bare metal against moisture quite effectively. If you use a lacquer-based primer and intend to wait awhile before top coating. A good sealer is highly recommended that seals against moisture.

[ronbarn]

This subject was reviewed on a previous thread, however, it is one of those which needs to be revisited periodically.<P>Conventional lacquer based primers are hygroscopic - they absorb moisture. The worst thing you can do is lay on a coat of that type of primer and leave it for an extended period without a finish coat - rust heaven and you have to start all over again.<P>Use a non-hygroscopic epoxy type primer (I prefer DP-40, but there are other brands) first. Read the instructions since there is a short window for coating over the primer without having to scuff it for the next coat. Since the epoxy primer I use has no filler value, I prefer to lay a coat of filler primer on top of the epoxy primer as soon as possible if I am going to leave the project for a while. That way the metal is protected and I don't have to scuff the epoxy when I get back to the project.