When using retarder with Vallejo Model Color do I still have to thin the paint,and also to say 10 drops of paint how many drops of retarder should I use??
Many Thanks for any help given

That's not an easy question to answer, & I'm by no means an expert.

The Retarder is designed to help the paint flow a little better, & to slow the drying time, so it can be worked a little longer, allowing blending, etc.

For brushing, I generally use a pallette with hollows in it, drip model color in it to about what I need, then add a little squeeze (1-2 'drops') to the pallette rim.
Then mix in a little of the retarder with the brush, untill it 'flows' with the brush, then thin with Model color thinners or water if needed.

For Airbrushing (MAir or thinned MColor) I generally add 2'drops' worth to about 3/4's of a/b cup, approx 20cc's.

Experiment & if you don't like it, Windolene with ammonia will get rid of it to have another4 go.

Thanks for your help Jon