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 ...... ?????

I have used elmers glue in a hypo to make rivets on the plug-worked really well-molds reflectedevery single one

I'm talking about P-51 flush rivets.

Today I just bought a variable temperature soldering iron to test it again. Maybe if I use it with a higher temperature it will burn the rivet ring this time.