Sounds like the dope is too thin if you are brushing it.

OR! Cover the undoped polyspan with SLC film. No dope required, no pinholes, lighter and stronger than doped polyspan by far. Scuff with steel wool and prime. No filler needed.

 This works for me when I get small pin holes in dope:  I take spackeling compound thinned with water & rub it over the areas, with a finger, that have pin holes in them. Wipe the areas off with a damp rag & let dry completely. Brush or spray over this area with dope & the pin holes are gone. I'm sure that talc could be substituted for the spackle, but I have not tried it. Its easy enough to try & it doesn't add any weight......Gene

Yeah Paul I know "exactly" what you are talking about.
I recently covered two models and both displayed these minute inconsistent pin holes after painting and they were not visible prior to paint application.
After exhausting research it was suggested to me that at sometime in the past I had sprayed a fish oil based RP-7 or WD-40 in the workshop and the fine mist had travelled (which it apparently does) to where I had the Aerospan (Polyspan) stored under a bench and it had not been protected.
No matter how well you clean the model prior to painting, there is no way the contaminate can be removed, as it impregnates into the fibres and lays in wait to repel the paint when applied.
Once done it cannot be rectified no matter how much you sand back the paint and re-apply a new coat - you have it for the life of the model !!!!
This answer doesn't help, but it does let you know to keep water displacing products out of the workshop and to keep your covering material "well" covered when not in use.
Cheers, Tony.

Larry,

Why not just cover it with the SLC film period, without overlaying the polyspan? The question results from the fact that I have never used either of them, but would like to know for the future.