Putty joints under linseed oil paint

[Fardelin]

I've put up some masonite boards and then fiberglass "reinforcement" at the seams. In one spot that I wanted to paint, I used some ordinary filler without thinking it through. When I then painted the wall, the filler "sucked" up the linseed oil paint like a sponge. This results in the paint being shiny on the masonite and very matte on the filled part.

How do I proceed with this wall, and how do I ensure it doesn't happen on the rest of the seams???

Oil filler? What type, where do I find it, etc.?
The idea is for the paint to be able to breathe, so no sealing of any kind.

 

[El Gringo]

Can't you saturate the filler with regular linseed oil first?
Otherwise, there is linseed oil filler you can use, but perhaps that needs to be saturated as well, fiberglass filler might be an option, it shouldn't absorb the oil at least!