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My wife's bike recently got scraped up and the paint got scratched down to the frame. It's an alloy frame. Should she be concerned about rust or any kind of damage? If so, do we need to get it repainted? There are about three scratches, each about half the size of a dime.

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Paint? Bikes have paint?? (If the scratches are down to the metal it's wise to cover them with something -- touch-up paint from an auto parts store, eg. But the world won't end if you don't do this -- the bike might become too rusty in 20 years vs 30.) –  Daniel R Hicks Nov 9 '11 at 22:11
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Aluminum won't rust. If your bike doesn't have dings and scratches, you're not riding it enough. :) –  Stephen Touset Nov 10 '11 at 0:22

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Chromoly can rust. Aluminum usually doesn't unless you're riding in the ocean. Cheapest fix is probably some automotive touch-up paint; sand / clean the area (scuff the surrounding paint up a little), apply a coat, wet sand, repeat. Won't be perfect but it'll protect the underlying metal from rusting.

Of course, if you want that perfect "unblemished" look you're probably going to have to plump for a professional (read: take your bike completely apart, sandblast frame, re-paint, put all components back on). Unless you're looking for an excuse to get a completely amazing custom paint job. At which point I'd recommend finding a good powder coat shop; my mountain bike's (chromoly frame) is powder coated and has held up really, really well over the years.

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So, are chromoly and aluminum alloy two different things? Her bike is listed as A-6 quaternary phase alloy. –  Joe Johnson 126 Nov 9 '11 at 20:29
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Is it an "A-6 quaternary phase aluminum alloy"? That's what most of the frames I've seen have. If it's an aluminum alloy I wouldn't worry about rusting (again, unless you're riding in the ocean). I think it's technically possible to have a quaternary iron phase alloy but it's almost impossible to generate the 1.21 jiggawatts to power it. Never mind getting it up to 88 mph. –  lawndartcatcher Nov 9 '11 at 21:23
    
Also (sorry - just re-read your question) chomoly and aluminum are different. Chrome / molybdium steel alloy vs. aluminum alloy (i.e., aluminum with other metals for hardness, flexibility, etc.) –  lawndartcatcher Nov 9 '11 at 21:25

I have had good luck with hobby paints.Most are enamal so they are durable,cost is generally under $2.00 U.S. for a 1/2 ounce bottle.You custom match your color by mixing colors in small amounts.My buddy has a Diamondback his color is three drops of blue with one drop of black. You can experiment to find the right combinations.Always allow the test sample to dry as the color may change,this can be speeded up by usung a hair dryer on low setting.Lightly sand the damaged area with very fine sandpaper,wipe it with a solvent (rubbing alcohol works fine).Apply the paint with an artist brush.If the area was the result of cables rubbing cover it with a patch of clear chainstay protecting tape.

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A cheap solution to stop the metal below the scratch corroding is to coat the scratches with clear / colourless nail varnish.

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