Thx guys, I used a combination of Tamiya Sand Weathering sticks and Weathering powders, as a rule of thumb I stuck the weathering stick onto a brush and worked the brush into all the nook and crannies (the weathering stick is thick and chunky so hard to use in nook and crannies), then with my hand I thumbed them clean just leaving the weathering stick gunk in the recesses, which helped as this stuff is a bit like oil or thick clumpy paint so it retains the weathering powders when applied over the top of it.
Then took the weathering powders and coated those areas again, I was fortunate that the original Marmit weathering had to be sanded off, so the scratched matte of the sanding on the armour, retained some of the weathering stick and powders naturally (once rubbed into sanded areas its incredibly hard to rub off), if you gonna weather your Medi's Id suggest sanding them, on the areas were you think they need it (ie. elbows, knees, arse, chest, etc) areas that in real life would get ware and tear with that armour on before applying your weathering materials
Then at the end I took a wet cloth and lightly wiped away the areas I wanted a bit cleaner, letting the water do the work (in some cases letting the water run over the armour) so it looked a little like they were caked in sand and then it rained on them
If you cant find the powders or simply dont want to spend money on them then you can try, grating chalk over sand paper to create your pile of powder ! I also took the powder onto the undersuit as that would retain sand in real life also
