js.b1 wrote:
First, check if you have water ingress into the boot (trunk if in
USA!).
Check the behind any lining at the back of the wheel arches for pools
of water - sunroof drain popped out of hole, electric aerial drain
popped out of hole, other leak (can need a hose to find it).

I think that's OK... I haven't seen any evidence of it (eg car easily steams
up inside).

Second, realise you can not solve this except by serious application
of new metal, but you can slow it down to something which is
controlled for about 3-4-5yrs depending on how severe it is.

Yes, it's all about control. I realise it's a losing battle eventually.

The best way to remove rust without removing metal is to use a 3M
Clean n Strip disc.

Are they really that much better than the 'wire brush on a drill shank'?
The latter I can buy cheaply locally.

Then you really want something electrochemical to prevent rust - sadly
most cars do not have galvanised wheel arches, those that do tend to
have very limited micron depth which erodes pretty easily. The problem
is, the more zinc in the treatment a) the more expensive b) the
heavier the can c) the more difficult to get cellulose paint to stick
to it! So generally go for the highest zinc content paint you can, you
then need a primer on top which permits car spray paint to stick to it
(galvanising metal primer). Car trailer people use Zinga or
Galvafroid, but that needs an alkyd paint (IIRC) on top or a
galvanising metal primer which is typically applied with a brush.
Pretty it is not, but it is very effective at slowing down corrosion
even from the other side (as it should be, I think 250ml of Galvafroid
is about £20 these days and weighs like a lump of lead likewise).

Thanks, useful to know. As this is only the exterior edges of the wheel
arches and a few scratches on panels, is this really worth doing? Water+mud
is more likely to collect inside. If I underseal the insides I can delay
that, but I think it'll still be the weak spot.

I probably have 100-200cm2 to paint, so this isn't a big job. The rest is
underseal time.

Theo