Flyingriki,
I had the plane about 4 yrs after using feather fill and didn't have any issues, but I live in Arizona. I have no experience with feather fill in wet climates.
Rich,
Tom,
I would fill the holes before flying. I did and was very pleased. It took some work using Superfill, West Systems epoxy and then a light grey primer. My painter was pleased because his concern was any oil getting into the fiberglass will cause problems later on. I also skim coated the inside of the cowling with epoxy for the same reasons.
I used the epoxy resin only method, which I read off of a Cozy builders website. I figured who better to follow than someone who has an entire aircraft of pinholes to fill. The suggested method was to brush on a coat of resin and let it set for a bit, then use a credit card or squeegee and scrap off all of the excess. This worked on my cowl like a champ. I figured if plain resin would not flow into and fill the pinholes, then how on earth would resin that was thickened with any media be able to get inside of a pinhole?