Series 3 Primer - small patches of bare metal

I am repainting my S3. It used to be a public authority vehicle painted bright yellow. The chap who bought it from the council obviously didn't like the yellow so painted it Limestone. Unfortunately he was ina bit of hurry and didn't do enough preparation and now the paint comes off when it's jet-washed. So I've rubbed it back to take off all the Limestone and am down to the yellow, or the old primer underneath the yellow.

However there are a lot of small areas where I've gone through to bare metal. I've seen plenty of advice on this forum that "you should always use etch primer on bare allumimium otherwise the paint will peel off". But I've also seen comments that etch primer over old paint will eat away the paint and it will come off. So do I just use an aerosol to pick out the bare bits?

The specification for U Pol #8 Etch primer (http://www.hex.co.uk/tds/upol/acid8.pdf) says that it can be used over "fully cured paint" - has anybody any experience of using it over old paint?

Spray test an area first then leave for a time to see if theres any reaction?
Ive used the ACID 8 BUT only on bare metal

Many years ago I stripped my series 1 86" to bare metal and left it for around 3 months then primed it with ordinary primer , the weather changes the surface and it will take paint without the acid etch.
I too on my series 3 have had issue with etch primer eating away good paint in the vicinity of repairs in future I am going to rub them back and let the weather do the work for me.

If they are small areas I personally wouldn't bother with an etch. As mentioned, I have seen etch primers take effect with local painted areas. Might be worth speaking to a bodyshop first to be sure.

I agree, I haven't yet used a primer or lacquer without creating the dreaded "oil painting effect" except for Halfords heavy duty clear lacquer.