No need for flash hobby grade stuff, there must be a shop near you that caters for home artists that like to paint portraits etc.
There is a product in aerosol can used to seal paintings.
I use a matt seal and its fantastic.
Once dry, THEN wash over the paint.
Im taking a guess at the spots of dirt on the tank being just as simple as tamiya Flat Earth XF-52 speckled on by blowing paint off a brush at a distance from the surface ?
Very nice job getting it all looking like you have

