Lee,
We have an Eriba caravan and run an Eriba owners web-site, and we see Eriba caravans from the 1950's through to brand new.
We recommend this procedure for the thorough cleaning of an aluminium and GRP bodied caravan:
Wash:
Using a good caravan shampoo to clean the caravan from top to bottom. (Fenwicks is highly rated, we use it, but we've also heard good reports of the Tesco "value" car shampoo).
Don't use a mixed "Wash and Wax" car shampoo - that's fine for routine cleaning - but you don't want a wax coat before you finish the cleaning process and it also hides surface imperfections.
Rinse:
Rinse the caravan to remove any soapy deposits - this is easier if you add a splash of wetting agent to the rinse water (Fenwicks Bobby Dazzler is good, but the car rinse aids are also OK)
Examination:
You should now have a clean van, with any black streaks or green growths removed.
Allow the caravan to dry - this lets you see if the surface is basically OK or if it has gone matt - and carefully examine the surface - and try to identify what the caravan is made from - painted aluminium panels, natural aluminium parts, moulded GRP (fibre-glass) components. Each of these will be further treated:
Painted Aluminium:
Take care, the paint on your caravan could be very thin and you should only polish this with a pure wax polish.
We've heard good reports of Mer, it is very easy to apply but it does have a very fine cutting action - hasn't been a problem but don't be to vigorous.
Rather better is a traditional paste car wax - like Turtle Car Wax in the small green tub. This has a much higher wax content and lasts considerably longer. But be prepared for quite a bit more physical activity on your part - it's quite a bit harder to polish off compared to Mer.
If the paint is matt, then a very light application of a cutting paste like T-cut will remove the oxidised layer - but be very careful.
Natural Aluminium Parts:
If the awning rail and other aluminium parts look a bit "snotty" then using a mild cutting paste like T-Cut will remove the bad surface.
The bare aluminium surface instantly creates a protective oxide layer, but you can further protect this with a coat of Turtle wax.
GRP mouldings and panels:
I expect the aluminium parts to be in good condition, but depending on how the van was stored - or if it has seen a lot of sun - then the GRP panels may be a bit dull.
Sunlight attacks the top layer of plastic and ultimately makes it look matt or even chalky. GRP is self coloured (unless someone has painted it) and the top 1 mm or 2 mm is pure resin (called Gel-coat) then the bulk of the plastic is reinforced with glass-fibre (hence glass reinforced plastic or GRP).
If the Gel-coat is matt and you want to make it shiny, you'll need to remove this layer with T-cut and you can safely do this on Gel-coat - each T-cut application will only remove a few thousandths of a mm.
If it's really chalky and the surface is flaking, then a more aggressive cutting agent (like very fine wet and dry paper) and a repaint with a GRP paint is required. On a 1990 van this is unlikely.
In any case a good waxing is needed afterwards to protect the surface.
Robert