Was the paint from can or actually done by gun? the problem could of beeen that you didnt let the paint flash off long enough between each coat, if that was the case the solvent in the paint was trying to escape through the next coat and that can cause bubbles.

also could have been an oily surface, a good product to use before spraying is called 'prepsol' but it may be called something different other there?

The problem is caused by the thinners in the primer reacting with the enamel paint.
The best way would be to either remove or seal the enamel paint. You could also just use enamel primer and paint. But if you already have your primer and paint and want the cheapest and bodgiest way to keep going. You will have to dust on the primer a few coats ( i mean dust it on lighter than light so the paint is almost dry before it hits the panel) then after 2 coats like that have dried you can hit it again properly. If you do it too wet or your paints too thin, it will go through and react again. .
Its only the thinners reacting and once it is dry it wont react, so you wont have to worry about the paint frying up in a few days time.

most primers will adhere to any surface (bare metal should be etched first). Your problem is either;
a) oil contamination - and it can simply be oils off your skin. always use a wax and grease remover (prep wash) before applying any paint.
b) you slapped it on too thick.

Youcan spray primer over enamel alright ,I think there is probably something else on the paint .There is a product on the market but I can't remember what it's called but it seals off everything and becomes a new layerthen you simply prime or putty over it .

Yo.

Firstly did you make sure the surface was free from oil substances before spraying on the primer.
Oh, if I were you, I would use paint stripper, get right down to metal and then primer.
Another thing, here in SA, we have a primer called MS PRimer{filler primer}, it'll help save time in regards to body prep, ie before painting. it gets rid of tiny marks etc, instead of using to much of spot putty.

Hope this help.

i have recently fixed some dings in my 1969 1000, i sprayed the bogged parts with primer, and all was well. But then to my dismay some little patches started to bubble up and peel. I am pretty sure this is because there is a layer of enamel in the original paintwork. is there a sealent or somthing that i can spray over my enamal (and acrylic) roof to allow me to prime it?
thanks