You'll most likely be chasing your own tail. Wet sanding w/ 600 grit will knock down the edges of your newly laid down paint, but it won't do much to hide the different shades unless you're talking about a small, small area. Like half-dollar sized spots. Then you'll have to run a buffer over your wet-sanded area.

As for "blending" w/o masking tape, don't bother. If your match is really that bad, you're just spreading out the eyesore. Plus when sun bakes on spray-paint can quality paint, it's gonna fade and look hellish after half a year.

Next time you attack a rust spot, knock out the rust w/ a small screwdriver. After the obvious cleaning, I've found that using a little paintbrush (like you used as a kid) and dabbing some primer followed by paint into the spot seems to stop the rust in most instances. Then if you want to hide your work, wet sand, buff, and then it should be fairly hard to spot your work from a couple feet away.

Next time find a quality paintshop who can better match your paint. They'll whip you up a custom can which really makes all the difference.

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'64 230SL