Frame Restoration

[divernick]

Please post some pics of this frame and the marks you mention.

[Caerus]

Any ideas or help please folks?

[EigerSA]

The brushed effect comes from the tubes being sanded so you could do it yourself but I would imagine it's a hell of a job, especially since the brushing runs perpendicular to the tube and not with it, could be done though,I'd save myself loads of hassle and just get it sprayed. As for the crazing, it looks like it's been plated with zinc or something and it's now starting to come apart, not saying that's what's happening, just looks like it... weird. Try sanding a spot there with 320 wet paper and soapy water, see whether it's just in the finish or deeper.

[Caerus]

The brushed effect comes from the tubes being sanded so you could do it yourself but I would imagine it's a hell of a job, especially since the brushing runs perpendicular to the tube and not with it, could be done though,I'd save myself loads of hassle and just get it sprayed. As for the crazing, it looks like it's been plated with zinc or something and it's now starting to come apart, not saying that's what's happening, just looks like it... weird. Try sanding a spot there with 320 wet paper and soapy water, see whether it's just in the finish or deeper.

 

I'll try sanding the fork a bit first, I may try a softer grit on the actual frame though, 320 seems quite rough. So you reckon go for a full respray, fork & frame? Any rough ideas what that would cost anyone? Its gonna break my heart to do it

[EigerSA]

320g is actually pretty fine. Remember that "brushed" look is actually scratches in the material, so you have to sand that out anyway. Just don't push like mad, let the paper do the work and use soapy water. As for cost, shouldn't be more than about R350 for frame and fork (powder coating). But if you going to get a respray, don't bother with sanding the frame, it gets dipped.

 

Can't really tell in the photo, but I don't think that it's proper "brushed alu", I'm pretty sure it may just be a paint effect.

 

this was started with 320 and finished with 1500

 

Edited January 18, 2011 by EigerSA

[Johan Bornman]

I've never seen marks like that on alu. Maybe a mutated woodworm? I guess the bike was contaminated by something chemical. But it is just a guess. It doesn't help, I know.

 

As for the sanding. Easy peasy. Sand it. If you don't like the effect, you simply change your method, the paper's grit or even paint over it. You can change the look from a satin finish with very, very fine paper, to something quite scratchy. Do experiment but remember, even that will require a coating - clearcoat from a panelbeater will work.

[Caerus]

I've never seen marks like that on alu. Maybe a mutated woodworm? I guess the bike was contaminated by something chemical. But it is just a guess. It doesn't help, I know.

 

As for the sanding. Easy peasy. Sand it. If you don't like the effect, you simply change your method, the paper's grit or even paint over it. You can change the look from a satin finish with very, very fine paper, to something quite scratchy. Do experiment but remember, even that will require a coating - clearcoat from a panelbeater will work.

 

 

The chemical contamination is possible, where or how I dont know. Ive just remembered seeing something like this a few years ago on some Ali we were fabricating. I guess I'll be sanding this weekend then. Thanks for the help. Any ideas on the best place in Durbs to get a decent spray job on the frame, will the forks need to be powder coated or sprayed?

 

Thanks for the help & info so far

[HeartCoppi]

This is filiform crevice corrosion on an alu surface under a coating like lacquer that has been breached.

It is common as no good priming adhesive has been applied.

 

Use steel wool dipped in paint stripper for starters.

Then start with wet 600 and progress through to 800 and 1200.

Leaving it raw is a bad idea: you'll get black marks each time you handle it.

Spray it properly.

[Johan Bornman]

Any spraypainter will be able to do a clearcoat job. However, frame painting is best taken to a specialist or, you take on the job of masking it. Panelbeaters don't think much of putting K2 on BB threads and inside headset seats.

 

To paint the fork, the best results can be had by first stripping the fork. The slider should ideally go to the painter completely stripped of all seals, fittings etc. Do mask the brake posts nicely so that you don't have to scrape paint off there afterwards. Dont get paint inside the seal glands. When painting the crown, don't forget to remove the crown race and do mask the steerer perfectly, right up to where it meets the crown.

 

Put lots of effort into the preparation and don't leave anything that requires intelligence for the painter to do.

[Johan Bornman]

This is filiform crevice corrosion on an alu surface under a coating like lacquer that has been breached.

It is common as no good priming adhesive has been applied.

 

 

 

Tell us more about this please.

[Caerus]

Any spraypainter will be able to do a clearcoat job. However, frame painting is best taken to a specialist or, you take on the job of masking it. Panelbeaters don't think much of putting K2 on BB threads and inside headset seats.

 

To paint the fork, the best results can be had by first stripping the fork. The slider should ideally go to the painter completely stripped of all seals, fittings etc. Do mask the brake posts nicely so that you don't have to scrape paint off there afterwards. Dont get paint inside the seal glands. When painting the crown, don't forget to remove the crown race and do mask the steerer perfectly, right up to where it meets the crown.

 

Put lots of effort into the preparation and don't leave anything that requires intelligence for the painter to do.

 

 

I think a specialist frame sprayer will be getting a call. I want to do a nice spray job. So you say spray the forks rather then powder coating? The forks are due a service(never been serviced in 8 years, and not one problem, amazing) anyway so, a complete strip down and spray on top of this should make them like new.

 

Thanks for the help & info, much appreciated all

[HeartCoppi]

If an electrolyte potential exists on aluminium surface it may result in corrosion. JB, I recall your active participation and photography of an interesting thread about a corroded BB in a carbon frame.

 

The oxygen content of the electrolyte (usually water (like water that gets inside a watch)) is consumed by the film reaction on the surface of the aluminium. The moisture at the mouth of the coating crevice is oxygen rich (cathode and alkaline) and at the bottom of the crevice it is oxygen poor (anode and acidic) thereby creating the required potential for a cell.

The pattern arises from the crevices or breaches in the coating.

[Caerus]

Tell us more about this please.

 

 

Filiform corrosion is a type of localized corrosion that is often associated with aluminum and magnesium alloys that have an organic coating. This type of corrosion has occurred on other coated metals such as zinc, iron and steel. Filiform corrosion tends to occur at high humidity, e.g. greater than about 75% and temperatures at or slightly above room temperature. The corrosion appears as thread-like filaments under the coating. The corrosion products cause the coating to bulge giving the surface the appearance akin to that of a lawn riddled with mole tunnels. The filaments proceed from points where the coating is no longer continuous. Numerous coating systems are susceptible. Condensates containing halides, sulfates, carbonates, or nitrates have been associated with filiform corrosion. Damage to the metal tends to be limited but the effect on appearance tends to be detrimental.

 

The mechanism of filiform corrosion is shown in the figure below.

 

 

filformmech-26140016.bmp

 

 

 

 

 

The mechanism has a number of characteristics that are similar to Crevice Corrosion, e.g. differential aeration and hydrolysis of metal ions resulting in increasing acidity in the region of dissolution. This type of corrosion has the following characteristics.

 

• The coating allows oxygen and water to migrate through it.
• The concentration of dissolved oxygen becomes highest at the back of the head near the region of the tail. This region becomes the cathode.
• Oxygen becomes depleted at the head. This region becomes the anode.
• Corrosion is driven by the potential difference between these regions, a potential difference which can rise to several tenths of a volt.
• Metal ion formation and dissolution proceeds at the head while oxygen is reduced closer to the tail.

Thus, the worm-like or thread-like structure that is formed has two parts that participate in the corrosion process, the front of the head in which metal dissolves and the region behind the head in which oxygen is reduced. Farther back is an inactive region in which metal oxide and metal hydroxide have formed a precipitate. Hydrogen bubbles can be formed if the head becomes very acidic. The propagating head region continues to move under the coating into new areas leaving behind a thin trail of corrosion under the coating. The threads can measure less than 1 millimeter across. Multiple threads or worms can appear under the coating. When two propagating heads meet, the propagation tends to stop. When a propagating head approaches the inactive tail it tends to be deflected.

 

The corroding alloy can affect the appearance of the filament. For example, filiform corrosion of aluminum or magnesium can result in a whitish precipitate forming in the tail. Filiform corrosion of iron can result in a head containing a greenish fluid (Fe(II)) and a tail containing a redish precipitate (Fe(III)).

 

The occurrence of filiform corrosion has been reported to be decreased by the following approaches.

 

• Application of more than one layer of a coating
• Use of a chromate containing conversion coating or primer on aluminum
• Use of a zinc containing primer on steel
• Reducing the relative humidity or maintaining a low relative humidity when storing items made of susceptible alloys.

 

The one thing that stands out for me is the bit about it happening at high humidity, which is why this has only happened since Ive move back to Durban 2 years ago.

Edited January 18, 2011 by Caerus

[Caerus]

If an electrolyte potential exists on aluminium surface it may result in corrosion. JB, I recall your active participation and photography of an interesting thread about a corroded BB in a carbon frame.

 

The oxygen content of the electrolyte (usually water (like water that gets inside a watch)) is consumed by the film reaction on the surface of the aluminium. The moisture at the mouth of the coating crevice is oxygen rich (cathode and alkaline) and at the bottom of the crevice it is oxygen poor (anode and acidic) thereby creating the required potential for a cell.

The pattern arises from the crevices or breaches in the coating.

 

 

What is your area of expertise HeartCoppi? That you know about this, is this part of your job?

[The Drongo]

Any ideas or help please folks?

 

@CAERUS

 

The marks are oxidisation. Pinholes in the clearcoat eventually allow air and moisture to get in between the cc and the ali. It then worms its way around. Quite a cool pattern actually.

 

If you are going to sand, the choice of grit is up to you.

 

You have two options.

1.Strip, sand and clearcoat (have a bodyshop do a good 2K clear)

2.Take the frame to a powdercoater. They will strip the clearcoat in an acid bath, and can then coat to the colour of your choice. I am having one done at the moment.

 

For option 2 you need to put in a bit of masking etc. effort.

Remove ALL bolts etc from the frame. Knock out headset cups. Ensure that all weld vent holes are open. ie not clogged with paint or dirt.

If your headtube is faced you can avoid re-facing by masking after they have dipped and prepped frame (before the powder is applied)

For your BB shell you will need 2 plate washers of slightly larger OD than BB shell -45mm will do)

A length of threadbar to suit the diameter of the centre hole (8/10mm) and 2 DOME nuts.

Place a plate washer either side of the BB shell. Threadbar through cut to size and nut either side to clamp. This keeps powder off the face and out of the threads.

 

I have them strip, then I collect and mask, or just do it there.

 

There are clear powders available, put the prep dipping usually leaves marking on the ali which does not look too cool. But speak to your local powdercoaters.

 

NB ONE THING. Powdercoaters who coat STEEL more often than aluminium have the ovens set to a higher temp. There is a posible danger that this can affect welding and even ali tube. A PC who works largely in ali would be best.

 

For your FORK you will have to do this by hand. I strip the fork completely (time for service!)

Remove the dust wipers. Then sand with waterpaper, hang lowers upsidedown using wire hooks through the bolt holes at the bottom, and then use rattle can HAMMERITE. Colours are limited though. White, Black, and used to be red, but have not seen it lately. You can spray HAMMERITE using a standard s/gun, in which case you can select tin colours. I find that it is more hard wearing than other paints. Only downside is that you will have to A. Do all 4 fine coats at once. +-15min intervals

B. You have to wait at least a week before working with it, as the HR takes time to cure. Any sooner and it marks. Even with finger pressure!

 

Hope that helps.

[The Drongo]

Any spraypainter will be able to do a clearcoat job. However, frame painting is best taken to a specialist or, you take on the job of masking it. Panelbeaters don't think much of putting K2 on BB threads and inside headset seats.

 

To paint the fork, the best results can be had by first stripping the fork. The slider should ideally go to the painter completely stripped of all seals, fittings etc. Do mask the brake posts nicely so that you don't have to scrape paint off there afterwards. Dont get paint inside the seal glands. When painting the crown, don't forget to remove the crown race and do mask the steerer perfectly, right up to where it meets the crown.

 

Put lots of effort into the preparation and don't leave anything that requires intelligence for the painter to do.

 

 

Touche'