>>6576242I like to do washes on top of gloss but the traditional approach to weathering in general seems to be to do it on top of satin (semi-gloss), see image for reference.
A final flat coat can affect the weathering, not sure what it will do to oils but it can certainly upset pigments or effects done by for example the Tamiya weathering master kits.
I would point you in the direction of the scale model general for weathering advice, just don't mention it's gunpla.
