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  1. #1
    Senior Member escarpment's Avatar
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    stripping a bonded al frame

    I would like to respray my 93 trek 1420. It is a bonded aluminum frame with about 1500 miles on it. From what I understand, sandblasting AL will compromise the structural integrity of the frame. And solvents are the safest way to strip paint. I dont have access to a sandblaster anyways nor do I think I could find one large enough. Im afraid to go the solvent route because of the bonding process. There is little documentation I can find on bonded aluminum but I believe the tubes are glued/bonded together with certain chemicals. I was told that it bonds at a molecular level but I am still skeptical to pour paint stripper on this frame. Anyone with experience. Thanks.

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    I have no experience with stripping, but I have 2 of these bikes myself. One has paint that looks brand new, on a bike I bought new 20 years ago, the other is pretty beat up, but I bought that one off ebay last year. As much as I'd like another one that looks new, I just wouldn't bother. How beat up is yours after only 1500 miles that you want a new paint job?

  3. #3
    Senior Member Ronno6's Avatar
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    Media blasting of aluminum frames need not be avoided, as long as the proper media is used.
    Sand is bad. That mostly applies to welded, heat treated frames such as Cannondales.
    If your Trek's tubing was heat treated prior to bonding, then sand as a media is definitely out.
    If the proper media such as ground walnut shells is employed, blasting is fine. There are others that are safe
    as well, but I am not sure what they are.
    I am not familiar with chemical stripping on bonded frames, but one could always sand the joint areas and
    chemically strip the lengths of straight tuning.

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    a77impala a77impala's Avatar
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    I had a Trek 6000 mtn bike I bought at a pawn shop, had really bad paint,all scratched up. I used
    stripper with no bad effects, then I polished the al, did not repaint. Have since traded it for a 930.
    Treks, 87-560, 90-930,92-970, 95-930, LeMonds, 2000 Zurich, 04 Tourmalet, 04 Arrivee, 06-Versailles

  5. #5
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    What's the problem with using sand as a blasting medium on some Alu frames - how does it affect the integrity of the metal?

  6. #6
    Senior Member Ronno6's Avatar
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    Quote Originally Posted by Continuity View Post
    What's the problem with using sand as a blasting medium on some Alu frames - how does it affect the integrity of the metal?
    Integrity is not really the property negatively affected by sandblasting. Although blasting with sand does remove more material than other media, that is not the only adverse effect.
    Blasting using sand as the media basically anneals the weldment, or in other words, reduces or eliminates the temper of the frame that was originally achieved through solution heat treating. Cannondale worked hard to perfect that process sooner than other manufacturers. That is why many manufacturers bonded their frames rather than weld them.
    Solution heat treating realigns the crystalline structure of the molecules; sandblasting alters that due to sand's sharp edges and hardness.
    Sandblasting can also result in causing critical dimensions and frame angles to go askew. Have you sighted down the
    tubes of a Cannondale frame? I doubt you've ever seen a straight one! That is the result of the hear treating process.
    Cannondale secures every frame in a fixture to hold all critical dimensions and angles during the HT process. Sandblasting
    can cause those to go awry.
    Glass bead, walnut shells and aluminum oxide are listed as acceptable alternatives to sand.

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    Quote Originally Posted by Ronno6 View Post
    Media blasting of aluminum frames need not be avoided, as long as the proper media is used.
    Sand is bad. That mostly applies to welded, heat treated frames such as Cannondales. If your Trek's tubing was heat treated prior to bonding, then sand as a media is definitely out. If the proper media such as ground walnut shells is employed, blasting is fine. There are others that are safe as well, but I am not sure what they are. I am not familiar with chemical stripping on bonded frames, but one could always sand the joint areas and chemically strip the lengths of straight tuning.
    Lets save the sand for blasting car frames and bridge steel. For most anything thin walled or soft, use soda or walnut shells for the media choice. Any competent shop will know this and recommend the right choice but ask before allowing them to start just in case that C word is not in the vocabulary...

  8. #8
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    That was most interesting - thanks for that, Ronno6.

    It still surprises me that sand will cause these effects, whereas glass beads etc., won't - is this because of sand's relative hardness as compared to the other materials I wonder?

    Quote Originally Posted by ksisler View Post
    ...just in case that C word is not in the vocabulary...
    Cannondale?

  9. #9
    Senior Member Ronno6's Avatar
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    Quote Originally Posted by Continuity View Post
    That was most interesting - thanks for that, Ronno6.

    It still surprises me that sand will cause these effects, whereas glass beads etc., won't - is this because of sand's relative hardness as compared to the other materials I wonder?
    Hardness combined with the sharpness of the sand grains.

  10. #10
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    Yes, your 1420 is a bonded frame and I would avoid all chemical paint strippers. There are paint strippers specifically made for aluminum so they won't hurt the tubes but I'd still be concerned about their effect on the bonds.

    The safest way to prepare it for repainting is hand sanding the old finish off with a fine grip paper. Yes, it will take time but won't damage the frame or it's bonds. I suppose media blasting with relatively soft media is OK but sand blasting is definitely too harsh.

    I bought one of these brand new in 1992, put 18,000 miles on it and it's now still being used by my son. I never repainted it and the factory finish is still decent but by no means pristine.

  11. #11
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    body man for over 30 years here.
    i have stripped several hundred aluminum car hoods with chemical aircraft stripper without problems.
    the problem with chemical stripper can get into cracks and other small areas and if not properly neutralized can cause problems later.
    i would avoid using aluminum oxide media because it is a very aggressive paint removal product. it can blast away aluminum in seconds. it works extremely well on rusted steel.
    i am surprised no one mentioned plastic media. i have had thousands of cars stripped with it without problems. if you are stripping only aluminum it would probably be the best stripping media.
    soda blasting could be another excellent stripping media. the only problem with soda blasting is it leaves a chemical residue. that if not removed paint won't stick properly.
    glass beads would also work also. just make sure you don't blast in one place to long. if you do it can be as bad as sand when blasting aluminum. glass beads are harder than aluminum and can remove material if not properly used.

  12. #12
    Senior Member escarpment's Avatar
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    Thank you guys for the information, While the paint is by no means in bad condition I would like to rid it of the trek logos and go to a solid color.

    I will look into sandblasting with something besides sand. I like the sound of the plastic media. As yes I am afraid of the chemical stripper getting in between cracks. This is my only road frame and I really love it. Even glass beads scare me, I used to work in a foundry and blasted many aluminum pieces and have seen how fast sand eats away at the metal. I guess I now have to find a business to do this for me.

    Or I will do it all by hand.


    Cheers.

  13. #13
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    One consideration before you go too nuts with the repaint. If you are the original owner and can document that (sales receipt, registration card, or other proof of purchase) Trek had a "lifetime" warrantee on these frames and will replace it at no charge if it fails at the bonds or cracks at the tubes and there is no evidence of collision damage. Those bonded Trek frames were pretty durable and the failure rate was relatively low but it wasn't zero so keeping it stock and keeping the warranty in effect may be worthwhile.

    Obviously they won't give you another frame just like it since they have been out of production for over 15 years but you will get the closest thing in their current product line. Just a thought.

    If you bought it used or can't prove you are the original purchaser, knock yourself out with the refinishing.

  14. #14
    Constant tinkerer FastJake's Avatar
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    Bonded = glued. Do not strip the paint off. If you really want to destroy any resale value of this frame by repainting it, just lightly scuff up the original paint with sandpaper and paint over that.
    Why "derailer" is the correct way to spell the gear-change mechanism: sheldonbrown.com/derailer.html

  15. #15
    Senior Member escarpment's Avatar
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    I am not the original owner, found it unused in an ex girlfriends uncles garage. It probably had >100 or so miles on it, original tires matching the rims still had the seams in the center. Nor do I ever plan on selling it.

    At this point I think I will do it by hand. Solvents are out and I dont feel like dealing with a sandblaster. ^None of this lightly scuffing though, Ill take it the the frame and repaint from there.

  16. #16
    Senior Member Retro Grouch's Avatar
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    Sammy Hagar 040.jpg

    For what it's worth, here's a data point. My fixed gear conversion started life as a Raleigh Technium mountain bike. It has an aluminum mainframe bonded to steel stays. I stripped it using paint stripper from the hardware store and painted it using spray cans.

    I wouldn't say that it gets a lot of use but it doesn't have any problems with the bonding or surface pitting or paint sticking either.

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