Spray Gun Paint

Yes, it will have to be thinned.

If you are using Testors enamel paint, you can thin it with Testors enamel thinner or mineral spirits that you can get this at your local hardware store.

How much thinner needs to be added? I've had experiences where too much thinner caused the paint to crack.

Is there a way to use that paint without adding extra thinner?

You really need to thin it. If you are worried about adding too much thinner then try 1 part thinner to 2 parts paint.

Rarely have I opened a bottle of Testor's/Model Master enamel that didn't require thinning, some of them have been thick enough to use as spackling.  So yeah, thin it down and spray it on.

If I add too much thinner, it is possible to make the paint crack once it dries right?

Not too sure on that. Just make sure you do not add too much. Stick with the mix I suggested, it is not too thin. If that will not spray then just add a few drops of thinner at a time to the mix until it works well for you. Paint some non critical things first to see what your mix does when dry if you are concerned. Try priming some cheap For-Sale signs and paint them if you dont have anything else.

I think that you might be worrying a little too much, this is probably easier than you are thinking. The best thing to do is simply jump in and learn by mistakes. I used to do kustom work on motorcycles and stuff and would be glad to help if you need it
Chris

Here is a bike that I painted

GTX, I gotta echo what the guys here are saying.  I love working with Testors paints and the stuff does need to be thinned.  I like to think of properly thinned paint as being like skim milk:  It's thin enough to run easily off the sides of the mixing cup but it's still opaque enough to not be "see through".  

As Compressorman stated, try not to overthink it too much.  Mixing paints for airbrushing is not an exact science and it can vary each time.  For me, I tend to thin to a 1:1 ratio of paint to thinner.  If I'm going to err, I'm going to do it on the side of having more paint than thinner.  Whenever I thin my paint for airbrushing, I make sure I have enough "trial and error" paint.  In other words, I dump in as much paint as I think I'll need and then I'll dump in even more so I can run some test sprays through my airbrush.  That'll help me determine if I've got the right mixture before I commit the paint to the plastic.  I just add a little bit more paint and a little bit more thinner until I get it just the way I like it.

Above all else, practice practice, practice!

Eric

GTX, the only times I've had cracking paint is when I've sprayed on too much at a time, when I've tried to cover acrylics with enamels or when there was too much of a temperature disparity between my spraying area and the drying area.   Oh, there was one time when the plastic had some weird mold release oils on it that reacted badly to the enamels giving me 'fish-eyes', so washing parts before hand is sometimes necessary.

Multiple light coats to build up the color is always better then too much at once- at the very least you'll avoid paint runs this way.  I try to have a lot of parts to be painted with whatever color I've mixed up so by the time I've finished the first light coat on everything, the parts that I painted first will flash-dried enough to handle a second coat.  Repeat until finished.

Like others have said, it is more a black art than an exact science so some experimentation is recommended.  

So far all the answers are good. Thinning is always a must when spray painting. However, the amount of thinner varies according to 1) your aperture on brush, .2, .3 or .5 usually. The finer the spray the thinner the paint to avoid clogging. Aaron Skinner has a basic video in How To section which gives a good starting point and a good tip to determine viscosity with a toothpick. There is a lot of trial and error here so start on scrap items and usage and practice will work it out. BTW a .3 mm is probably ideal for most work in scale models. the .5 will take less thinning but eat you paint supply.

Good luck

It needs to be thinned,I usually just eyeball it to the consistancey of skim milk

that is a very nice paint job!

Thanks for all your responses guys. Ya, I see that it's trial and error. I'm just gonna keep working at it.

I'm gonna try to follow some of your ideas.

I'll try to post some pics of what I do later on.