Russian steel strikes again. My father bought an Alfa, and as far as we could tell ( he was an automotive engineer) it was made from compressed rust. The right time frame for that hot rolled steel that had so many rolled in slag impurities that it would rust even under paint. Solution after you have finished reinforcing it is to seal it with boiled linseed oil, diluted 50% in illuminating parffin and sprayed inside the box sections, and painted on a thin film outside, followed by a 2 week stay in the sun to allow it to drip mostly out. Messy, but I bought a Mazda with a little rust on the panels barely starting to bubble through, and did this to it, and left it in the garage with cardboard under the sides to collect the drips. When I sold it 10 years later, after having driven it to the ocean and the dam with a boat, launching with it going in the water on a slipway, the little bubbles were still there and no bigger.
As to the Sinclair GRP body, first thing is to wipe down with a solution of household bleach and water 50/50. Do this outside after removing all steel parts, and tape over all decals you want to keep. Use good gloves as well. Then rinse and dry. To clean the badly degraded GRP use a cloth and household scouring powder ( I used VIM, should be a similar white powder there in AUS from Lever) to remove the degraded top surface. Then use car burnishing paste, which you get from your local motor paint supplier, to buff the remaining surface to a shine. Then wax and polish with real car paste wax. Works a charm.
Plastic go get some Dash 20 silicone polish, used for detailing cars, and wipe over. You can do everything with this including the GRP, but note it makes it slippery, and a seat done with this has absolutely no friction. I went for a few swims with this, but the boat looked great and looked like it came off the showroom floor. If you want I will decant around 500ml and post it to you, as I buy it in 5l bottles, and it last a really long time.