yeah, another weathering question!

If your finish is flat, try some chalk dust. Get some pastel chalk from just about any art supply store (don't get the oil-based chalk, just plain old dusty chalk). Rub it on a piece of fine-grit sandpaper, run a fine paint brush through the dust, and then "Paint" it on the surface. It will stay pretty decently on a flat surface, but not worth a hoot on a glossy one.

Try it on some scrap first until you find the look you want. It will come off, but on a flat surface it gets down in the grain and can be tough to get rid of completely.

Are you after 'streaks', or 'stains'?

Two different looks requiring two different methods of application.

A streak is more defined, being obvious to the eye, a stain starts out as a heavy blotch at its point of origin, and filters down to a light 'stained' look, following the flow of water etc.

For a streak, just paint light strokes as required, using acrylics and allowing a few minutes drying between strokes.

For a stain, place a drop of colour where the rusty point is, and allow it to dry.
Afterwards, take a clean, fine brush of decent quality, and dip it in some thinners, you want the brush to be 'moist', not sopping wet!

Work in gentle strokes following the flow of water, and work in that direction only.
Gently draw the brush from the paint downwards, then wipe your brush, and do it again.

The thinner will slowly reacticate the dry spot you made, and allow for light amounts of the paint to flow with the brush.
Stop when you acheive the look your after.
BTW, oil paints work best for this.

Hope this helps!

thanks guys...i'll give each a try! :)

I have what I call my chunky wash. It's something that I think just got created from adding more paint or thinner to a wash over time. I use it around fiuid fill areas and for streaks. i just start with a drop at the top and drag ait down the side useing less and less of the brush as I do. Seems to work for me.

Polly Scale water based paints have a "rust" color. If you dilute it, it is a pretty convincing rust. I always use a diluted "oily black" from the same company over it in different areas to give the streaked look. Over lacquer finishes, one can use a pecil eraser or a clean toothpick and 'scrub' away this paint if you overdo it or to streak it more noticably.