@cavestone I also use vallejo paints. The key question here is are you brush painting, or using an airbrush, or both?
The Vallejo paints have a “Model Air” line that comes pre-thinned. You can buy a thinning product (from them) to add to the “Model Color” series paints, which are the exact same thing, just more concentrated and thicker.
If you’ve never used an airbrush, its a bit finicky–it requires regular cleaning and a without a good brush and compressor the paint will flow very inconsistently. The process will be very frustrating unless you use the pre-thinned paints because the brush can only blow paint that is within a certain set of thickness parameters that are difficult to achieve, especially if you are trying to mix the air in the brush’s reserve or in small quantities. That’s why having copies of both Model Air and Model Color is fine by me–unlike Warhammer 40K I’m using primarily earthy colors, greens, dunkelgelb and dunkelgrau… other AFV and camoflage colors and so i don’t need a collection of primary or bright colors.
But I recommend it. The airbrush creates a sublime, beautiful layer and slight mix variations can be used to create shadows, grades, etc. Then you go back over with your brush, adding highlights, lines, details. Then, back over again with a drybrushing…then back to the airbrush to blast Flory Wash all over it.
Can’t see your photos, you can’t post photos until you have a certain post count. I wouldn’t use water to thin paints, they are water soluble but the mix and viscosity is all wrong (though water is obv. cheap and easy to clean up)
As for oil-based paints, these require thinner to clean up, they are much more difficult to drybrush with and are not used for airbrushing. The days of the old testors paints is long behind us–even apple barrel paints are easier to work with and cheaper. The early painting methods were much more about glomming thick layers of paint onto lead miniatures and the modern approach with water based paint and a series of different techniques creates much better results.