I'm not certain on this, but I think the "soft" pastels are the
oil type pastels and as such unusable for the type of weathering
you're speaking of. Chalk pastels are just as the name
implies...chalk. Grind alittle off against sandpaper and they
work fantastic. I've used them to weather vehicles and as shading
on figures. Seal your kit with dull coat to protect your finshed
paint job...do your weathering then seal again to protect the
pastel. You can always apply more pastels and reseal to create
the look you're going for. When finished seal with dull coat or
what ever finish you're looking to have, glossy or otheriwise.
One of the nice things about using pastels is until you seal it,
IF you don't like what you've done you can remove it with a Qtip
and some water. Also, you might want to test your pastels first,
some brands darken more than others after sealing. Experiment
alittle to find the look you want.
Hope this helps...
Rogue