Aluminium Roof Rack

[Peter Connan]

Gareth, I would recommend leaving the holes. You will cause more issues trying to fill them than what the holes cause.

[Steele]

Hi Peter, why do you recommend against filling the holes? I was planning to use putty or filler of some kind if I am going to paint over it. For powdercoating I have no idea how I would fill the holes.

Anybody know what powdercoating costs these days?

Mods - I wonder if maybe you could move this thread to the vehicle accessories section? Gone way past selling roof racks now

[Tinus lotz]

Durram looks legit . I dont like alli coulour podercoating flakes to easy

[Peter Connan]

Hi Peter, why do you recommend against filling the holes? I was planning to use putty or filler of some kind if I am going to paint over it. For powdercoating I have no idea how I would fill the holes.

I assumed you meant welding them up. That would create soft areas.
With putty or filler, sure, but I don't see the reason. If you really want to do it, try Pratley Steel Putty?

[Steele]

Yeah, I remember from my days doing NDT that alu welding would not be an option. The problem with the holes is that they haven't been drilled straight which means the bolts used have pulled the alu resulting in raised, rough edges. I'm going to have to hammer and file these edges down and then want to fill the holes to prevent chafing and damage to whatever I carry on the roof rack.

[mvcoller]

Much faster and cheaper than painting. Buy a sheet of roof plastic from local hardware and lay it on the garage floor after packing a square of bricks on the floor big enough to fit the roof rack in.

Then drape the plastic on the floor over the bricks. This will be your "tank"

BLACKENING ALUMINUM
by
JEROME KIMBERLIN

There are several ways to colour aluminium black and among them are black anodizing and paint. You could rub dirt into the aluminium surface, I suppose, but of all the methods, I think chemical colouring is the superior method. It is certainly cheaper, faster, and home use allows the enthusiast greater flexibility in the timing of his decoration of work in progress.

Surface preparation of parts to be coloured black is all important as any irregularities are not covered by this finish. Paint does build up and fill in scratches and other voids. Castings, however, should look like castings if the prototype used castings, so surface finish is always adjustable to the builder’s idea. The point here is to emphasize that this blackening technique will not cover up mistakes.

You will need three chemicals. These are: Nitric Acid, Copper Nitrate, and Potassium Permanganate. You will also need some good quality water - either distilled or deionised.

Take: Water 750 ml
Add Nitric Acid 5 ml
Add Copper Nitrate 25 gm
Add Potassium Permanganate 10 gm
Add Water to make 1 litre

Obviously you will have to make up more or less solution to fill the container you will use to colour aluminium parts and the parts to be coloured should be completely covered by the solution. You should use a glass or plastic container. A metal container will poison the solution prematurely. Scale this up maybe 20 fold to get the right depth for your roof rack. Turn the roof rack over to do the other side, it only take 15 minutes per side and comes out beautifully even, no messy paint jobs.....

At 24 C temperature, the blackening process will take about 15 minutes using a fresh solution. If it takes longer it means the solution is deficient in one of the components. Usually, copper nitrate and nitric acid need be added.

Aluminium is a strange metal to most of us. While we cannot see it, the surface of a newly machined or cleaned piece of aluminium combines with oxygen in the air to form a self protecting coating of aluminium oxide. This happens within minutes. If this surface continues to grow (get thicker) the blackening solution described here will not work satisfactorily. Thus, the piece to be coloured should be cleaned just before immersing into the colouring solution. In my experience, glass bead blasting is a superior way to clean the aluminium surface and the choice of bead size determines surface finish. Once the bead blasting has been accomplished, the beads can be washed off with hot water and the aluminium piece immersed in the blackening solution. I recommend that the time between blasting (cleaning) and immersion in the blackening solution be less than two hours. I once waited five hours and was disappointed in the results. Once the blackening process has been completed, wash off the work piece with tap water, drain and spray with WD-40 or other water displacing oil.

There are a number of ways to clean aluminium satisfactorily. It is possible to simply sand the surface clean, or scrub it clean with an abrasive. One can also chemically clean aluminium by degreasing the work piece then dipping it into lye (Draino, for instance) for a few minutes or seconds as required, then rinsing. The shape of the work piece and the model engineer's facilities often dictate what method of surface preparation will be used.

Model engineers wishing to use this solution to blacken aluminium castings or other parts should be aware that the chemical components may be hazardous. While the solution itself is not particularly dangerous it can make your hands purple, so use rubber or plastic gloves. Potassium Permanganate is classified as an oxidizer even though diluted solutions of it are used throughout the world to sterilize vegetables used in salads, etc. Concentrated nitric acid is just plain bad. The technique for using it is to pour out a little in a glass container and then use an eye dropper to transfer the liquid to a measuring container when the volume wanted is small, such as that described here. Nitric acid also turns your hands yellow, hurts, and removes fingerprints. A good way to avoid eye damage is to wear a face shield such as the one you should be wearing when working in front of your grinder.

[Spike]

Yeah, I remember from my days doing NDT that alu welding would not be an option. The problem with the holes is that they haven't been drilled straight which means the bolts used have pulled the alu resulting in raised, rough edges. I'm going to have to hammer and file these edges down and then want to fill the holes to prevent chafing and damage to whatever I carry on the roof rack.

I closed the holes on my rack with a hot-glue gun. In fact the hot-glue gun has become my fix everything tool

[Steele]

Thanks Malcolm - that looks very interesting. Only downside is that I will need to get the current powdercoating blasted off before doing this.

Spike - I would never have thought of that in a hundred years!

[martyn]

The etching/anodizing won't work on the areas that you repair if the repair hasn't been done with aluminium welding.