Pastels sticking

i usually don't put a barrier coat between acrylics and washes.  Tamiya, MMA, Vallejo paints washed with , oils, weathering powders, pastels, acrylic washes. never had a problem.

You're brave. I've had 91% Iso take off cured acrylic paint many times. Tamiya/Gunze. Heck, even Mr. Surfacer if you use enough Iso. Maybe 70% would be less harmful?

Mustang

I hear ya on that and the clear has to be cured. I use MM acrylic flat as a barrier to do my oil washes and DOT filters on armor and then followed by the alcohol treatment with MMP Powders. I can see that being a problem if the clear acrylic flat has not fully cured. I wait several days to let the clear cure and I apply it without rubbing or going over it multiple times.

I coat the model with a Vallejo matte varnish, which is impervious to IPA or typical acrylic thinners, and then use Tamiya acrylic thinner to apply pigments.

plasticjunkie

I have used rubbing alcohol in the past,

I have used rubbing alcohol in the past, depending how thick or runny you want your application. It dries up really quick and then use a firm brush to adjust the effects. The alcohol will make it adhere even better.  

Chrisk-k

A fixer doesn't bond pigments to a surface; it helps pigments flow into micro crevices on a matte surface.  Thus, you need to apply pigments to a matte surface.  A glossy surface doesn't have micro crevices in which pigments can get stuck and is not good for pigment application.  Try to apply pigments to glass.

...which then helps it adhere. If he's still having trouble on a matte surface, I'd suggest a fixer. 

A fixer doesn't bond pigments to a surface; it helps pigments flow into micro crevices on a matte surface.  Thus, you need to apply pigments to a matte surface.  A glossy surface doesn't have micro crevices in which pigments can get stuck and is not good for pigment application.  Try to apply pigments to glass.

Use a fixer, such as mineral spirits, or the pre made ones from Mig or AK. Youtube is your friend for pigment how to's.

That's about it J. I usually dry brush the ground up pastels on in layers if I need to in order to darken the areas and feather/ fade them out like on exhaust stains. Sorry dude that's about it from this corner.