Hello Harry!
Yes, I meant a transparent varnish, mat or gloss - it's your choice. Now that I look at the pics for the N-th time I think I can describe it a bit better. The weathering you have on your model now looks like dust deposits that would be OK on a tank - sitting in one place for a longer time. Now a moving tank, as I imagine 'em, has a little less dust on the front and a little more on the rear, where the dust cloud from the tracks starts to sit down. Also the horizontal surfaces should be more dusty and the vertical surfaces less dusty.
Now when the weather is wet, it all changes - mud and rain make a vehicle look so that it's almost clean on the roof, and 100% mud colour around the wheels, and the transition is very gradual with the exception of places that wear out because of being touched contineously. Water streaks can also be visible. Airbrush is best for getting a model to look like that. You can also get nice realistic streaks using paint thinner on fresh paint, barely dry from airbrushing.
I hope it helps you - good luck with your Sherman and have a nice day
Paweł