Hi Paul,
many thanks for your feedback, but I had a 'free' day today and the weather was quite mild so I was keen to crack on whilst my motivation was high.
I found a couple of tubes of this stuff in my 'come-in-handy-one-day' box and a quick check on the manufacturers website confirmed it was suitable for bonding GRP to steel (amongst many other things). I can't remember why I bought it but its seemed to be suitable so I've used it on the first side.
Polyurethane Adhesive/filler by
Sabrebuilder, on Flickr
The first side is setting over night so I'll see how strong it is tomorrow.....
....peter
P.S. I've watched your car building and I'm amazed at what you achieved on a slopping front drive. It's a great looking little car, well done!!