Well yellow is a color that is naturally very transparent. First off, never, ever, never, ever apply a "heavy" coat of paint. It does nothing but add weight. Always apply very "light" coats with about 10-15 minutes "flash" time in between each light coat.
You must seal the wood in the cockpit area before you apply a primer or paint. Primer does not do a very good job of sealing the balsa and you will not be happy with the results. There are products out there to seal balsa, I like to use Zap finishing resin.
You didn't answer the question, what color Solartex are you applying the cub yellow over?