Thanks so much everyone for this info. I took your advice and did some tests using the products I already have at home.
I cut an A4 sheet of 250gr Bristol Board into 6 pieces and I sprayed Com-Art paints on to each piece in the same patterns. I used Com-Art Opaque Black and White then 3 Transparent colours and let them dry for a couple of days. One piece was left as a control the other 5 pieces I sprayed (Eclipse HP-SBS 0.3mm at 30psi) with 2 light coats of different products, I used a hairdryer to dry in between coats and then left them for 3 days to really dry. I then lightly rubbed each piece with a white slightly damp cloth. Here are the results;
Schmincke Aero Medium - rubbed down to the paint layer and damaged the paint.
Trident Transparent Base - rubbed down to the paint layer and damaged the paint but took a bit longer.
Golden Airbrush Transparent Extender - rubbed down to the paint layer and damaged the paint.
Golden Polymer Varnish with UVLS - Satin - left a matt finish and fully protected the paint, no damage.
Golden Polymer Varnish with UVLS - Gloss - left a slight gloss finish and fully protected the paint, no damage.
Conclusion: The Varnish worked very well and the other products did not work at all as a protective layer. To get a true Gloss or Satin finish I would spray more thin layers of varnish till the desired effect is reached (let each layer dry before the next application). With the Satin varnish, 2 coats left a good Matt finish and was enough to protect the paint from damage. Mixing the prescribed 2:1 Varnish to Bottled water sprayed very well although next time I might try a bit more air pressure for even better atomisation. Spraying light layers did not re-wet the Art-Com paint but care is needed to not spray too thickly which could do damage. Spray from 20cm distance.
The Varnish did make the card curl up so to counter act this you need to spray a coat of varnish on the back as well.
I prefer using the Golden Varnish rather than Spray Cans, as it is Water Based, I can mix to whatever reduction I choose, I can mix the Satin and Gloss varnish together to get a different effect (I did that on my brush paintings) and there is no Can to get rid of. Golden Polymer Varnish comes in a plastic bottle, is water resistant, flexible and protects from UV rays and dusting.
I read somewhere that Fixative is simply watered down/reduced Varnish so that should work fine although I did not try it.