RE: Zlin Z-50 Nitro Models
The Chinakote has a few problems as you've indicated.
I've run into this as well... especially once the plane has a few flights ( and fuel ) on it.
There is an easy solution though.
I've found that you can spray the plane either before or after you built it it with Topflight Clear Kote.
A few passes will give you a shiny and SEALED finish that holds up very well.