Will pastels fade or become transulcent when adding a gloss coat over them.

I use pastels often for rust, exhaust and fuel stains. When you seal it some will blow off and lighten. You may want to go extra heavy or, like me, be happy with a more subtle effect.

Yes,the clear coats will diminish them,I never feel a need to seal them,I do it and avoid handling them.

For exhaust you can also use actual charcoal powder, which clear coats don't fade. Artist's version available at art and craft stores.

You can also make powder from regular barbecue charcoal briquettes, which is my source; one will last a lifetime! I just sand a little off as needed with an emory board/sanding stick -- just make sure it's plain charcoal, and NOT the 'easy light' kind with the starter chemical built in. (I haven't tried that version -- but I'm assuming whatever that stuff is...it can't be much good for plastic models or paint. )

I usually do this. I use the least pressure from the max distance. I DO NOT spray directly st the surface either. Slightly off center. THIS HAS WORKED FOR ME FOR YEARS.

 Also I Use Craftint Clear as it seems to have a lighter "Touch" when going on!

Stuart06

I want to add some exhaust stains and don't want to use my airbrush as I can't get an accurate placement of the exhaust stain out of the engine manifolds.  I want to use pastels as I find that would be easier to brush on and can customize the color of the mixture with multiple colors and layers.

I don't want to use AK pencils, but feel I can get a better effect with a masscara tip brush.

However my fear is that once complete and I want to seal the model with a gloss coat, the pastels will dillute and become invisible. 

Generally, yes, a sealer coat will reduce the intensity of the colors, though as someone else noted, black itself is practically unaffected.

As was also mentioned, you can address this by applying the chalks to anticipate this effect.  I will apply a color perhaps a little darker or heavier, knowing that my sealer coat will reduce the effect a little.

You don't need to seal pastels after you apply them, though, unless you plan on handling your model and touching those areas where you applied them.

And as far as the specific product to use is concerned, I bought myself a set of a dozen pastel chalk bars almost 20 years ago, at an art supply store, for ten bucks.  They're earth tones, shades of brown from dark to light, and a reddish brown, with a yellow and a black.  Ten bucks.  I scrape the bar with a knife to get powder and apply it with an old brush.  I refuse to pay Mig or Mr. Tamiya for something I can do myself.

I also use powdered chalk to make things like mud on armor, by mixing the powdered chalk with a drop of water and a drop of dishwashing liquid.  I apply this to the model and when it dries, it looks like mud.  Varying the color can help make it look fresh or dried.  I can also mix some chopped twine with it, to look like grass or straw, or some dried tea, to look like leaf matter.  You can play with the mix and see what you can come up with.

Hope that helps!

Best regards,

Brad