If you get the paint to the consistency of skimmed milk using water you will find it flows perfectly from any decent sable brush.
Artist mediums are ofc just as viable, I just don't see the point in spending money on things you can essentially get for more or less nothing.
But maybe you should try them if you are having trouble just using water... you might find they really make a difference for you, in which case there is nothing wrong with that.
The thing to remember is that if you can see the undercoat a little through the coat you are applying it is no bad thing. Just wait for it to dry and apply another coat until it gives you a good even coat. This will also help remove 'brushstrokes'. Of course in some situations, strokes can work, it depends on your style. Patience is the key, and persistence will get you everywhere when it comes to painting.
I do blend a lot with my style of painting, but I just work quickly when I do it, otherwise your paint will dry up. But then this can be resolved by using a decent palate that has no absorbent qualities.
- Cover the model in Silver spray paint.Why would you do that?
Okay. My method would be as following:
- Black undercoat sprayed on. (make sure you do it about 30 cm away and in short bursts so to get thin coats rather than have it pool in areas). Now most of your model has black parts, so you use this as your basis for working up... doing a silver layer would be pointless.
- Paint the gun boltgun metal mixed with chaos black. (or just silver then wash heavily with badab black).
- Block out the armour plates with the blue you want as the mid-tone
- Highlight the black areas with a grey (dark then light in two thin respective coats)
- Wash the blue armour with badab black
- Re-layer the original midtone blue but avoid the crevasses in the armour etc. So only the raised areas
- Add bleached bone to the blue (about 70-30) and fine edge highlight the armour
- Do a further highlight of 50-50 mix and only apply it to the very highest raised areas and corners.
- Mix boltgun metal with mithril silver 50-50 and highlight the gun (avoiding getting any in the recesses)
- Fine edge the gun with mithril silver
- Use what ever colours you wish to pick out the details.
This will give you a very effective paint scheme and look good.
You don't have to do the 2nd highlights, but I think they are always worth the effort. GW minis aren't cheap and for me it's always worth putting extra effort into them

Hope this helps you out anyway. Let me know how you get on regardless
