My 5x7 Norma back had this corrosion in the light trap channels. A strip of Scotchbrite wrapped over the end of a screwdriver cleaned it off nicely. I re-painted (with Airfix modelling paint, matt black) and the corrosion hasn't come back.
Aluminium in dry, clean air grows an oxide which is thin, even, hard and impermeable. With electrochemistry you can make the layer thicker, and colour it, which is what anodising does, but again, you usually get a well-behaved even layer. The thick, crusty oxides can be created by acids or alkalis, or by prolonged contact with salt water, sulphur compounds (Al foil over eggs is instructive), and all manner of more agressive chemicals.
I suspect my back had been in damp storage for a long time. The light trap channels had a black cord to provide extra light sealing, and they would have trapped moisture too. Whether the ions that damaged the Al surface were already there in the paint or primer, or whether they were in the atmosphere, is impossible to say.
There are Al-specific primers and paints if you want to re-finish your Al after cleaning off the crud, but I would reckon it is only necessary in the light traps and internal blackened surfaces. It is well worth cleaning off the crumbly flakey crud, because it will trap moisture, and it probably contains the ions which are actively breaking down the Al surface. Keep warm and dry thereafter.
Al lens barrels can end up looking equally unsightly - I have a wide angle rectilinear where the Al cells are impossible to remove from the brass barrel because this kind of corrosion has locked them in place.
PS: I don't know the 8x10 models, but the smaller format Normas have machined locating lugs to provide a reference surface which defines the rear focal plane. This is the only part I would worry about cleaning too agressively.
Reply With Quote
Originally Posted by Jac@stafford.net
Bookmarks