I don't have first-hand experience, but I asked this question myself just yesterday to Tom Ellis, the sole remaining enamel guru at Thompson Enamels.  He suggested Thompson's 1030, he said it was acid resistant.  I assume that since both leaded and unleaded can fire on top of unleaded, that you could use whatever you wanted over it.

Lillian

thank you for your response. I will try the product you mention.

MR

Hi MR,

I thought that the Foundation White is what I was suppose to use as a white first coat.  I find that I am not getting a good coverage with the Foundation, so I have been using the titanium white and now I get a great consistent white canvas to start working on.  These are Thompson enamels.

I am rather new too, and I don't usually "chime in", but this has been something I have recently been dealing with.

Virginia

Thank you Virginia.  I will try titanium white.

MR

What is the best enamel white of the brands to use for enamel on copper under other opaques or transparents?

Thank you,

Mary Rose