to put the top coat on it won't guarantee the job. It's all about paint "systems". The primer has to be compatible with the top coat in order for everything to bond together. Different shops use different "systems" i.e. brands. PLUS, the comment above about primer being "hydroscopic" means it absorbs water which means it will have microscopic rust growing all over it if too much time passes between primer and top coat. Your Z will be a bubbling mass of shit eventually.