Hi aidan , Ian has got all the bases covered , all i can add is to repeat some of those points . Like anyone who sprays cars will testify , the surface preparation is the one thing that makes the biggest difference to the final finish. The plastic should be primed , then the primer can be wet sanded with the finest grade you can muster (but definitely not polished!, that would only cause problems with paint adhesion). Its then a case of applying the gloss finish as carefully as you can and make sure theres no chance of any dust etc. getting in it while its wet. Im not a fan of polishing paints on a model as i think it can cause more problems such as going through the colour or fine scratching plus the problem of getting into hard to reach areas and the polishing medium needing cleaning out of the nooks and crannies! If you do get some dust/flaws in the final finish , i prefer to gently flat them out , again with the finest grade of paper you can find (if necessary try getting 2 pieces of 1200 grit wet and dry and rub them together to lessen the abrasiveness ) sometimes a little soap or washing up liquid helps to keep the paper off the surface a bit ,again lessening the cutting effect on the paint. Then when the flaws are smooth , another quite well thinned coat of the finish paint misted on to restore the shine , and repeated if necessary. As for the mixing in the cup, thats exactly what i do , putting the thinners in first then the paint , but always give a little test squirt first so you dont spray any neat thinners that may have got into the nozzle onto the model. cheers tony