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I completely sandpapered my bike a few months back and repainted it with a coat of primer first, and then matt black. The paint was sprayed only after sandpapering the bike with the appopriate gritt so as to make sure that the primer would stick great.

With the original paint, my bike hardly got scraped too much and too badly, but now, it is easily scratchable. The pure metal is visible as if the primer and the matt black simply were peeled off (where the bike got scratched).

What could I have done so differently to make it so much more scratchable than the original factory paint job?

More Info

  • I sandpapered the bike clean and wiped it down with rubbing alcohol as suggested (I don't remember where).
  • Avoided touching the frame before the painting.
  • Sprayed a coat of primer.
  • Once dried, sprayed the matt black paint.
  • The paints were not at all sprayed heavily to make the paint peal off, and not too lightly either.
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  • 2
    What paint did you use?
    – andy256
    Apr 16, 2014 at 13:18
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    Many (most or all?) factory bikes are powder coated. Read up on Wikipedia for more informaiton.
    – Kibbee
    Apr 16, 2014 at 13:21
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    @Kibbee I don't think that's true for most road bikes and I know quite a number of touring and mountain bikes that are painted instead of powdered.
    – arne
    Apr 16, 2014 at 13:51
  • You may have used an incompatible primer. Is the bicycle steel?
    – Batman
    Apr 16, 2014 at 15:42
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    This is what paint does. Depends a lot on the quality of the paint, but pretty much all spray-can paint is pretty soft. It does, however, harden over time -- so after maybe 3 weeks it should be harder than after the first few days. But if the paint and primer are coming off and leaving bare metal you probably used the wrong primer, or applied it incorrectly. Apr 16, 2014 at 19:40

5 Answers 5

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Commercial paint jobs often use heat-baked enamel, with multiple coats (not just undercoat and topcoat), and finished with a clear top coat.

The baking process produces a really tough, well bonded coating, and the clear topcoat (as well as being tough) produces a nice finish that hides scratches.

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    And many are painted, as @Kibbee says, by the powder coating process.
    – andy256
    Apr 16, 2014 at 13:24
  • Hmm, this does make sense... Thanks @andy256!
    – Ihsan
    Apr 16, 2014 at 16:34
  • Is this necessarily true for 2k enamels? I thought they were just dried at about 85F for several days? As far as I'm aware, only when frames were dipped in natural oils and gilsonite, as well as more recently powder coated, are they ever out in an oven. Mar 6 at 3:32
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Since OP said the bike was Aluminum, we should note some things:

  • Painting aluminum almost always goes wrong when done at home.

  • Upon sanding, often sand particles are embedded in the aluminum (sand blasting likely doesn't help). Chemicals are the way to go for stripping aluminum frames.

  • You need a different type of primer (etching primers specifically intended for aluminum), such as this Rust-Oleum Automotive etching primer. You typically follow this up with another compatible primer.

  • Powder coating / doing this professionally will probably the way to get it to actually work.

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Been there, done it all. I've been airbrushing and painting cars, motorcycles, helicopters, helmets, zippy lighters... and just about anything since I was a kid. I could always draw like I invented it, and hated the fact that my work had to be hidden in filing cabinets after all that effort or else the graphite would smear. So, a more durable genre developed.

You're experiencing what stumps most people when they start... and that is paint chemistry. I had to take classes on it specifically in college, not to mention spend years learning from the established guys, and my own trial and error.

You've got an aluminum frame? If your bike is new and you're changing the paint scheme, remove all the decals, and lightly sand the existing finish. It's the best bond you're likely to have, and the existing finish is a fine foundation. Then, a coat of sealer, which basically just means to bring the bike to a uniform color. Black or white is usually fine depending on what your final color will be. White for a light color? Black if it'll be dark. Only really exception is candy paint (silver for cool candy colors, gold for warm colors).

If it needs to be taken to bare metal...fine. Don't sand it though, you'll be there all week and it's a mess. Instead, go to a local carwash, soak it with "Easy Off Aerosol Oven Cleaner" and wait 15-20 minutes. Have a beer. Then, pressure wash it. BOOM, bare metal. That easy. (OH, don't wear flip flops.😬)

Next is paintint. Most automotive finishes are two-part paints with a catalyst and a harder. You want a 2-part etching primer for bare metal. It's available in cans with a res button on the bottom. Slapping it on a tabletop pops a bladder inside so the two mix. Shake like hell for 10 minutes, and follow directions. Here's a satin black from Eastwood https://www.eastwood.com/2k-aero-spray-rat-rod-satin-black.html

Color and all that can easily be applied with whatever paint you want. Testers model paints even has a flip-flop color shift in an aerosol can. But you can use createx airbrush colors through a cheap gun... or whatever. The sky's the limit.

Finally, add decals if you like (sets available on ebay) and then use the same 2 part puncture can for the application of the gloss clear. BOOM. DONE. https://www.amazon.com/USC-Spraymax-Matte-Clearcoat-3680065/dp/B0178ABUVM/ref=asc_df_B0178ABUVM/

Keep in mind that flat and matte paints are nearly always powder coated or baked on. For your flat black... I'd go to Harbor Freight and pick up the powder coat gun and a bottle of flat black....then get industrious with some scrapped kitchen ovens and make a little enclosure to bake them in. But that's me.

Good luck.

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  • Welcome to the site - excellent first answer. Keep it up!
    – Criggie
    Mar 5 at 18:20
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I've painted a number of bikes now and I always run into the same problem. I was advised to use etching paint on bare metal before priming, but this did not help. It's really frustrating because you spend a lot of time preparing the surface and painting, then the paint scratches the first time something hard (or even not so hard...I got a scratch just accidentally banging the bike frame against a wooden door) touches the new paint job.

I've come to the conclusion that this is just the way it is with home-made paint jobs.

If I was restoring a really nice/expensive bike, I would get it professionally painted, but this reduces the amount of pride you have for you bike.

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    Completely disagree - this should not be "the way it is for home-paint jobs" Rather its a failure of the paint system. eg: I have a spray can of car exhaust manifold matt black paint. On handlebars etc, looks like normal paint but flakes off on the first gust of wind, or a sharp look. This paint requires baking "through normal operation of the engine" so I use a cheap home hot air gun to heat the part up to 150-200 degrees C , and its totally like rock after cooling. A light sand, coat of black gloss spraypaint and left for three days and the part looks new.
    – Criggie
    Jun 23, 2016 at 23:05
  • Surely using a hot air gun would just remove the paint ? As is one of the uses of these guns?? Jun 14, 2020 at 7:12
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    @RoisinEllison Possibly - it depends on the paint. I used engine paint which needs hundreds of degrees C to set. A power-coat would also be set by heat and not come off with hot air. Common aerosol paint would probably soften and come off with heat, but a 2 part spray mix would not. It all comes down to what paint, how it was applied, and cured, and surface preparation. Oh and sometimes there's a clear lacquer on top for protection, which should be "compatible" with the underlying layers otherwise it comes off really easily. A good clearcoat helps durability too.
    – Criggie
    Jun 14, 2020 at 10:59
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    "This is the way it is for DIY paint jobs", @Criggie, because most people don't DIY it correctly. Certainly sounds like you do it right. ;)
    – FreeMan
    Mar 10 at 19:20
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There are several key elements you need to consider for a durable paint job for all types of work.

  • Binder Type: This is the "body" of the coating. If you cast a thin layer of Jell-O around your bike frame, its probably not going to last. In order from generally most durable and adhering to least, you've got a choice of ceramic, epoxy, urethane, polyester (alkyd), and natural oils. It should be noted that the top 3 typically require mixing with a catalyst that is very harmful to health without proper PPE. Most spray paint is likely a polyester mixed with solvents.

  • Timing: Ideally, you will apply thin coats after the solvent has evaporated off, and ambient oxygen begins to help cross link some of the molecule chains within the binder, but you also want your new coat to bind with it. Very ideally, you wait an entire day between coats or more, sand the coat, and apply a new coat. This ensures a greater cure for each coat. Because the slight polymerization and solvent flashing will shrink the coating, adding a new coat too soon will cause some stress and separation between coats. Additionally, for non-catalyzed coatings the full cure can take nearly 30 days. Colder temps mean longer flash and cure times.

  • Surface grit: Your bare metal should be between 220-440 grit (American). The UK grit sizes are completely different, and the rest of the world uses micron. A more viscous paint should have a larger grit (lower number), and thinner paints use finer grits. This ensures that no air is trapped in pores, but also that coat is minimally thick to a smooth finish. Additionally, sandblasting produces the best finish for adherence, but the type of blast media can vary depending on metal and paint.

  • Surface cleanliness: There are oils in your hands, on your sandpaper, in the air, and just about everywhere else. Since your paint is only microns thick, having a micron of non-hardening oils can really make your paint softer and less "sticky". Use an evaporating, non oil-based degreaser/surface cleaner to remove oils. Dish soap is just oil with a surfactant and will not work.

  • Temperatures: Typical best temperatures are between 75-85F to paint. That's just how the engineers made it.

  • Clear coat: Most top coats (those paints with pigments) are made to be durable, but not have much in the way of ultraviolet ray resistance. A clear coat will generally add toughness to scratch prevention down to your primer, as well as prevent fading and cracking due to UV and weather exposure outside.

  • Paint Pigment Quantity: This is very case specific to the type of pigment, but the more pigment you add, the less binder there is per unit volume, so the weaker it becomes. Not really something to worry about unless you are tinting paints yourself.

This comes from experience and research into paint chemistry. I've had massive success with ceramic coatings under the Cerakote brand. I built an oven out of two filing cabinets a bolted together with a toaster oven slotted into the bottom. After baking for a very thinly sprayed coating for 2 hours, I accidentally dropped my frame from 3 feet directly onto concrete, landing on the bottom bracket shell. I dusted off the concrete that scraped onto the frame and it looked brand new. Rather than scratch my bike, the bike actually scratched the floor.

I've not done as much work with epoxy, but if you can't bake your parts then this is what you should use for the best primer. It has the lowest UV resistance, but provides best adhesion and hardness. You can use an epoxy topcoat, but for purely protecting your bare metal you do not need as tough topcoats.

There is also the choice of paint application, which varies by binder type. Brushing is the worst because it produces very thick coats that do not cure uniformly. Powder-coating (and baking) works well for multiple binders because it coats extremely thickly but also completely cures it evenly, producing a very thick and smooth shield. Ceramic coatings must be sprayed very thinly and baked to leave mostly just ceramic, which is thin enough to flex with the substrate without breaking, and adheres almost as well as the metal does to itself, but the coating will show existing scratches very easily. Some manufacturers, like All-city, will deposit the paint particles electrically while the frame and paint are in a liquid bath. This is about the same durability as powder-coating, but requires fewer human interaction. Paint sprayers can also be set up to electrically deposit for better performance, but at high danger to the people spraying.

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