You can use/do washes over a matte base, I frequently/typically do…..it is however not advised, and can make the wash or filter much more difficult to control and work with, as the W&F will leech/seep into the “pores” (potentially making it look “speckled”) and spread (wick) on the matte surface, making hard to control..and almost impossible to “work” or remove without leaving stains/traces on the surface.
Acrylic washes are also more complicated and tricky to work with than oils, as they do not have a lot of “working time”…..they dry fast, and once dry…that’s pretty much it (there are ways to deal with this such as adding a retarder,…and some tricks you can use to touch-up/repair small areas.) The benefit of the Acrylic though is that it dries dead flat…which is great for rust and dust.
Oils paints inherently allow for much greater working time and control…you can often come back hours or a day later and still work them to some degree. When talking about oils, you also need to differentiate, between “Artists Oils” , Humbrol (or other white spirit thinned hobby paints) and specially mixed/premixed weathering products such as dust washes and streaking-components. The latter all are generally made (or certain ones in the line are) to be matte, however, the artists oils are a bit more tricky, because they generally use linseed oil as a binder/carrier for the pigment…the linseed oil is what can cause a sheen to the paint when applied. Some paints are worse than others (my personal preference are the ABT-502 Oils)….however, this can be somewhat mitigated/eliminated, by placing a small amount of the paint colour(s) that you will be using on a piece of ‘Chip Board’ (other cardboard works also..I just have a pref. for Chip Board)….let this sit for about 15 mins before using the paint, and this will then wick some of that excess linseed oil out of the paint. (you will notice with some experience/experimenting that there is a Sweet spot for this…where the paint still spreads and flows well without a lot of pigment speckling/separation when applied, and between when it does begins to do so.)
[In my own builds, when I work over matte paint, I frequently apply oil based filters over the paint as a first step…this gives me a slightly satin/”eggshell” surface. (after a week or so of drying)I will then often follow this with some panel shading/fading, or otherwise colour adjustments or additional modulation using oil based paint…this further slightly enhances the satin surface….this satin finish then later on allows the pin washes to flow like I want them to.]
Many of the modellers that I know like to use a satin finish (be it the paint itself, or a clear coat) to applly oil washes and filters over. This satin surface allows the washes to flow and be better controlled (and touched-up) as needed.
One thing that does concern me when I read your post/approach, is the use of oil based effects (washes/filter, etc) over an oil based base coat/color…this is pretty much a no-no.
The very basic/general rule of thumb in this is to, work opposite materials/finishes over each other. For example:
Acrylic Base > Acrylic or Oil Based washes/filters
Acrylic Base > Acrylic Clear > Acrylic or Oil Based washes/filters
Acrylic Base > Oil Based Clear > Acrylic washes/filters
Oil Base > Acrylic washes/filters
Oil Base > Acrylic Sealer > Oil Based washes/filters
Oil Base > Oil Clear > Acrylic washes/filters
..not all these are options I would use...and they can get more complex when you start to do special weathering effects such as HS chipping....but it is the general concept/approach.
You want the sealer to protect/encapsulate the material/paint below. As long as you obtain full coverage of the area/surface/piece that you spray it will be "sealed". The important thing is the full coverage, and choosing the proper material/paint for this that will work with the material/paint you are sealing, and will work with any subsequent material/paint you apply over it.
Note also that some of these paints that you are using/mentioned are not “hobby paints” and they may have chemicals in them, or drying time/out-gassing issues, etc., that do not react well with certain hobby paints you may apply over them. They may also not allow for certain hobby paints to properly bind with them when applied over….thus causing lifting/peeling down the line….so make sure you experiment with them first.
Insofar as what to use for a satin varnish over your oil based paints,…It needs to be an acrylic based one…but which mfr./brand is a personal choice. I personally have been using the Vallejo “Satin” clear, or if I feel daring, Tamiya ‘flat clear’, with some ‘gloss clear’ added. Both of these will need to be Airbrushed though, as they do not come in cans.