An update, just in case anyone else has this issue - I spoke to B&O about this; in summary they said that the platters originally had a clear-coat sprayed on them, and that this had probably been damaged and that the only way to fix it would be to get the platter re-coated, but that's not a service they offered, and they don't stock spares (no surprise there). So I decided to experiment!

I suspected that the clear coat had actually worn off some time ago (the platter is 30-odd years old and any coating is unlikely to have survived that long), and that the white was actually light corrosion of the Aluminium as a result of the loss of the clear-coat protection; so I thought i'd approach the restoration with that in mind.

Rather than use Brasso (as suggested by another forum member) I opted for something a bit less harsh - A mild silver polish I have previously used to good effect on cutlery (The can said it also worked on aluminium). Wow - What a difference! Starting gently at first, and interspersing with cleaning in cold Soapy water (and drying thoroughly), the marks gradually faded to the point where I have a platter that looks pretty good. The black markings on the platter also seem to be unaffected by the cleaning.

I'm sure further cleaning would improve the result even more (there are a couple of areas that still aren't 100%), but i'm not going to push my luck and risk ruining it.

I anticipate that the platter will degrade again over time, without it's protective coating (i'm tempted to seal it, but not sure what with, as I don't want to use something that  will end up damaging the vinyl); hopefully it will do so gradually and more evenly than it did last time (which I suspect was as a result of a previous owner leaving the platter wet). In any event I'd be more than happy to repeat this process, given the result.

We have had several threads about this subject.

B&O platters have a coating that eliminates static electricity build up on the record as it rotates and plays.
This coating will be irrepairably damaged by washing with water or solvents.

Use a soft dry cloth to wipe the platter - nothing else.

You are washing away the coating.
If you continue to wash and rub, you will eventually have removed it all and eliminated the platters
antistatic properties completely.
And yes, depending on the records you play, you can hear the difference very clear in the form of static discharges ("pops").
In severe cases it may even cause the thin coil leads inside the cartridge to burn over (which usually means goodbye cartridge or at least a costly repair).

Never clean B&O platters with anything but a soft and dry cloth.

Martin

I will seriously consider getting the platter re-coated now though (B&O recommended getting it re-sprayed with a clear coat at a local paint-shop!) - just need to research options 1st.

It is interesting though that I've noticed the BeoGram deck suffers much less from static build-up than my Rega deck (You can lift up the anti-static mat on that after playing an LP!), even without the anti-static coating.

I agree 100% with Martin, once you wash the coating off with either water or solvent, you have effectively ruined the platter and will need to buy another.

Thanks for the info Martin - It's an interesting theory, though i'm not convinced that the vacuum created by the spinning of a record would achieve any meaningful degree of down-force, the forces invoked would be miniscule - sounds like the usual Audiophile/B&O snake-oil to me, of which i'm very sceptical [:)

Fortunately, it seems my platter still discharges, since it doesn't seem to be building up any significant charge; certainly nothing like my Rega -  I'm not surprised by this though, as the gap between the streaks and the platter is nothing like big enough to prevent static leakage.

For the sake of purism, I might see if i can get it coated with an antistatic clear coat; there are a few on the market.

Thanks for your help/info.

If you are used to other brands, I fully understand your sceptism, but B&O never sold snake-oil.
- And never claimed to be "Audiophile" in any way.

For selected units B&O released a limited number of technical manuals explaining the theory behind the units, the mechanics and electronics, - often down to component level. For their own educational purposes I believe - some of them clearly typed on a machine with added
handdrawings etc. and then duplicated. Some were called "Teknisk Produkt Information".
I remember seeing the downforce calculations in one of them (Beogram 4000 perhaps) and even if I don't remember the
exact amount, I do remember being quite surprised by the amount myself. 

Martin

Unfortunately I think it was ruined before I got my hands on it; so trying to make the best of a bad job. I'm pretty sure its salvageable though.