paint advice needed for me , a total paint newbee.

You can paint over Hammerite with any synthetic but it would be more compatible if you paint in a synthetic primer and bin the Hammerite.

You don't have to strip all the paint off, only remove lose or flaking paint and provide a key by rubbing it down. Synthetic paint will NOT react when painted on top of any existing paint finish but it WILL react if other non-synthetic paints are applied over the top of it, like Hammerite.

Have a look at this data sheet:

you'll find Tekaloiddo a nice Alkyd "Red Oxide" (BA74 R01) synthetic primer and you can savethe Hammerite for railing, drainpipes etc. Steve.

thanks for the info steve. unfortuantly i have to use the hammerite as im already behind on this and im pressing to get it done (the rebuild) before christmass so waiting on a delivery (can only be done on saturday) will set me back another week and the rebuild is pretty much waiting on the bulkhead being installed. as its only the bulkhead , most of which is hidden it shouldent be to much of a problem if the finish isent up to scratch. im still in two minds though with stripping the vehicle with nitromores before painting , the rear tub is pretty badly scratched and the bonnet has been partly painted with what looks to have been an old rag. all three doors will be new as will the seat box so all i will have to strip is the tub , bonnet and sides (window'd) and the two wings. i understand i will need an etch primer but will the bare aluminium need prepped to accept the primer in any way?

also , after painting im storing the panels in my shed , would keeping a heater on in there help the paint to harden?

thanks for the info steve. unfortuantly i have to use the hammerite as im already behind on this and im pressing to get it done (the rebuild) before christmass so waiting on a delivery (can only be done on saturday) will set me back another week and the rebuild is pretty much waiting on the bulkhead being installed. as its only the bulkhead , most of which is hidden it shouldent be to much of a problem if the finish isent up to scratch. im still in two minds though with stripping the vehicle with nitromores before painting , the rear tub is pretty badly scratched and the bonnet has been partly painted with what looks to have been an old rag. all three doors will be new as will the seat box so all i will have to strip is the tub , bonnet and sides (window'd) and the two wings. i understand i will need an etch primer but will the bare aluminium need prepped to accept the primer in any way?

also , after painting im storing the panels in my shed , would keeping a heater on in there help the paint to harden?

when i did the 101, the instructions on the etch primer said to key the surface before painting it. I think i went over it with some coarse wet and dry before spraying it with the etch primer.

if it doesnt have much paint on then go ahead with the nitromors. if it has a lot of paint on (like 10 layers of military paint then it can take years!)

just bring them into the house for a bit ;) (says he with a living room full of wires and dashboards...)

If the paint is particulary bad then you could strip it although this is time consuming, expensive and messy, or you could use an electric/air sander to remove all the paint defects more efficiently.

Acid etch is required to give your primer something to bite into because Aluminium (birmabrite) is a natural oily metal which has very little adhesion quality. You can use a scotch-cloth (medium grade) to scuff over the bare alluminium which will provide a key for the etch primer.

It wont make that much difference, heat will only help it dry quicker, air flow and light will help harden the synthetic.

Steve.

Isn't it because the oxide on aluminium is a slippery bugger, since the oxide is a very hard ceramic.

I've never seen any metal described as "naturally oily" before

Steve

The oxidisation or ceramic as you call it is a natural process.

Aluminium is classed as a reactive metal and like iron it reacts with oxygen causing surface corrosion. Aluminium left in its bare metal state corrodes producing ?aluminium oxide? which is a form of corrosion that will completely cover the surface and yet protects it from further corrosion. This white aluminium oxide powder formed on bare aluminium can be scraped off, which will lead to further corrosion because the oxide is naturally weak having little or no strength.

This process is called anodising.

However anodising aluminium, by an electro chemical process for example, physically alters the metal surface and produces a really tough dense layer of oxide offering maximum protection to bare aluminium that does NOT need painting.

Without the anodising process aluminium remains oily.

Steve.

Stephen Hull wrote: This white aluminium oxide powder formed on bare aluminium

I've a feeling that´s actually aluminium hydroxide ( Al(OH)3 ), not aluminium oxide (Al2O3), its not actually strongly bonded at all. Aluminium oxide is clear as far as I can remember. Out of interest, undoped Alumina is the gemstone corundum. Doped, it becomes "sapphire" (add titanium) or "Ruby" (add chromium). Alumina IS a "ceramic".

No, that white stuff is purely a corrosion product. Its anodising when its grown on the surface electrically, usually in a sulphuric acid process, sometimes in a chromic acid process. Its anodising because the job forms the positive electrode of the cell.

What is "oily" ? do you mean "non-stick¨ ? Oily to me would imply it has lubrificity, and it doesn´t, aluminium seizes and galls very easily in screws, and anything you accidentally stick together before you get the clearances right.

Steve

I know it's off topic but there seem to be knowledgeable people here, what's the green coating often seen on alloy parts inside aircraft to prevent corrosion, which seems to work pretty well, is it some sort of anodising?. Greg

I think thats a chromate passivation, and its done by immersion in a really nasty mixture of phosphoric acid, hydrofluoric and chromic acid, but since it needs hexavalent chromium you ain't going to see that in Europe anymore , under RoHS regulations.

Steve

Probably a process known as Alocrom, which is a chromate conversion. There are several different types available. I think it is Alocrom 1000 that produces the greenish tint. 1200 is more of a gold colour.

I think there is a bit of a debate at the moment about the legality of 1000 due to the emission of hexavalent chromium being at about the EU limit.

Regards Jeff

We used Alocrom a lot at the last place I worked but it was golden, I didn't reaslise there were variations. It was of very variable quality, sometimes it would wipe off on your hands!, but I put that down to crap suppliers.

Greg

I agree, although I was only generalising but the corrosion product acts as a form of natural anodising that helps protect the bare alloy yet its not really anodised in the true sense of the word. Aluminium is also classed as a reactive metal and you would expect bare aluminium to corrode quicker than it actually does yet it does not because the formation of this white powdery aluminium oxide on the surface protects the metal to some degree as a sort of anodisation but it is very weak.

IIRC electro-chemical anodising is a process which creates a tough dense hydroxide layer that resists abrasions and helps protect the aluminium underneath. The chemicals involved in anodising are indeed Sulphuric acid, Sodium Hydroxide and Sodium Sulphate.

IKWYM, cutting through a Land Rover body for example is extremely difficult, almost impossible with a hacksaw.

I'm not qualified to comment on the chemistry of aluminium only paint (allegedly) and from a painters point of view, bare aluminium is described as an oily metal in terms of paint adhesion which is where acid etching comes into play by getting a paint substrate to stick to it.

Regards,

Steve.

There is really only electrochemistry going on. The sodium hydroxide is used to etch a nice surface on the metal before you start, the Sulphuric acid provides a conductive path for the electricity during the anodise, and I don´t recognise sodium sulphate as part of the process. Oxygen is released at the anode terminal of the system (ie the job) and that´s what makes the oxide of course.

Its actually very easy to anodise things, if you feel like having a go. I did some small parts at work this week, and dyed them a fetching deep black. Once the part is boiled, the resulting coat is VERY tough (and its the hydrated form, as you say)

Thats because its almost fully annealed isn´t it ? So soft it sticks very well to your saw.

Steve