can model master airbrush thinner be use to thin model master flat clear lacquer finish? not sure if this thinner is compatibile.
i also have normal generic brand mineral spirits. dont remember what i used last time to thin this stuff though!

I would have to say no.

Model Master Thinner is an Enamel while the Clear Flat is Lacquer based. Go to your local DIY center and pick up a bottle of lacquer thinner to thin the Flat with.

im assuming that mineral spirits is also enamel based ?

I have used generic hardware store PAINT THINNER to thin Testors' Dullcote and Glosscote for airbrush use for decades on hudreds of models and figures with no problems. A few drops in the color cup with the clear and I'm good to go. I have sprayed this over ModelMaster enamel and acryl, PollyS/PollyScale, Tamiya, Andrea, Vallejo and Citadel acrylics, artist oils, new and old formula Floquil, as well as a few other brands that have come and gone.

First of all, if you are mixing brands and solvents, make sure that you test your mix on something other than your model before you destroy a lot of work.
That being said, as a rule, try to use the same brand and style of thinner as the material that you are mixing. It can save a lot of heart ache down the road that is often not worth saving a dollar or two on a bottle of thinner.
Regarding Model Master thinner in their flat, you might contact Testors directly on their web site for the best answer.
Mineral spirits is used to thin oil paints.
Enamel reducer is used to thin enamel paints.
Lacquer thinner is used to thin lacquers, but can be used in enamels to speed drying time. Lacquer thinner has the potential to attack enamel paint and plastic if sprayed too wet and heavy. Quality lacquer thinners are sold with different drying times and called "hot" when they have very fast drying times. This is better for plastic models and over enamel, because the solvents evaporate faster and have less time to attack the plastic or enamel under the paint.
Purchasing generic "paint thinner" at a hardware store or general craft store is kind of hit-or-miss, unless it specifically says what it is intended for. It is easier and cheaper, but make sure that you do a test spray before you use it on a finished model.