You can apply the panel line wash over the top of a Matt varnish... but it works best (and is easier to clean up again) if you use either a gloss or semi gloss varnish as a base instead. With the Matt varnish, you risk staining the paintwork around the panel lines, as the wash will tend to soak into the grain of the matt paint, which is near impossible to remove again. (Gloss has little to no grain which allows you to wipe any excess wash away again) In saying that, this effect is sometimes desirable, depending on what you want to achieve etc.
You also greatly reduce the risk of getting "silvering" when decals are applied over a clear gloss varnish. Just ee sure to seal them under another gloss coat before applying a wash to prevent any seepage from running beneath them.
For what it's worth, I always seal my paint (and decals) with Tamiya X-22 clear gloss (mixed 50/50 with Tamiya lacquer thinner) then apply a panel line wash (usually enamel based) then, after cleaning it up, I finish with a final clear coat of Tamiya XF-86 Flat clear... again mixed 50/50 with lacquer thinners.
You can achieve a more "Satin" finish from the flat clear by diluting it with more thinner in a ratio like 25% paint to 75% thinner, and applying it in a very light coats to gradually break down the glossy sheen etc.
Try a small section on the underside of the fuselage first to see how you go rather than doing the whole thing at once. That way you won't run the risk of ruining it if it doesn't work out to your liking.
24. August, 21:29