Day 1:I applied oil pin washes with odorless mineral spirits.

Day 4: All the spots on the model touched with the mineral spirits were still satin. Anyway, I then applied a Vallejo Matte varnish, thinking that it will make the spots look flat.

Day 7 (today): The spots are still satin! Despite flat-coating, one can still see all the spots touched by mineral spirits.

Has anyone had this kind of problem with mineral spirits? Can 2nd flat coating solve the problem?

In many cases, getting a total flat finish, requires multiple
applications of flat varnish.

Hope this helps,
Cheers
Joe

Also make sure that whatever mineral spirits you used aren't wicking off the flat clear coat.

I don't know what "odorless" mineral spirits actually is, or how it reacts with oils. I pinwash with the same type paint the model is painted with, only flat black thinned with real spirits for running into seams and creases..never have a problem..
I would think that a coat of flat clearcoat would kill the shine no matter what..unless it's so glossy the clear is beading off and not sticking to it. that would present a problem.
I hate mixed media...I never use it.

I think it has something to do with odorless mineral spirits. I'll switch to regular mineral spirits.