Erik,
I wouldn't use acrylics to create streaks; use oils or mig enamel products instead. When using oils, use something like mig enamel thinner to thin and streak the oils (I used turpentine and it was quite strong, causing the varnish coats to become mushy). I paint with tamiya acrylics, and seal it up using testors dull coat (the dull cote in rectangular bottle with pink label, thinned with testors lacquer thinner - cylinder bottle with black label). Then I typically begin weathering with a wash around details, then some oil paint streaks and fading, and finally pigments. The testors dullcote stands up to these products, thinners, and pigment fixers. Only problem I've ever had is when I used turpentine with the oils, which is why I switched to mig enamel thinner. You could also try AK thinner. Also, I recommend abteilung 502 oil paints, but if you don't want to order, the local art shop should have Winsor Newton "Winton" oil paints. I've heard that Wilder weathering oils and Van Gogh oil paints are good (never used them myself).
Kien