What's the best primer to use on a mold....??

I'm having a hard time to make flush rivets on my mold.

I have primed a fiberglass fuselage with 04 coats of light polyester primer but can't make the rivets using the soldering iron technique.

The primer is not soft enought as it's not being melted by the soldering iron.

I'm using a 25W soldering iron.

Any ideas / advices ...... ?????