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If you will stick to 1/72 the smallish top cups will in all likelihood always hold sufficient paint. Howsomever moving up to 1/48 or 1/32 you will find that 1 or 2mL of paint will prove to be insufficient. My 1/48 B-29 easily consumed an entire jar, 23mL, of Flat Aluminium paint and my present 1/48 Chinook easily used 10mL. Additionally the ability to use suction feed means that you can simply attach the jar that the paint came in using it as your feed cup giving you less to clean up saving all that wasted paint and money.Never know if I will need to repaint my truck?
I do 1/48 exclusively and the size of the cup has never been an issue.
I totally agree with Geo. I started using Vallejo Model Air about a year ago and am very pleased with it right out of the bottle with no thinning. My only problem with Vallejo is that their Air colors don't match their Model colors with the same English name. Other than that I use Tamiya acrylic paints, in general, and Testors' Model Master acrylic line for specific colors like German RLM colors. I never mix my own as I'm a lifetime member of the Close Enough SchoolVallejo Model Air. These spray fine right out of the bottle
Using Turbo's method would perhaps be doable (spraying directly into the cup) but I cringe at the clean up!
Also, thanks for the photo attachment Turbo. It is a rare image of a model in this stage of the build. I suppose you create your own canopy masking (Tamiya yellow tape?)?