Magic restorers are a waste of time.
You need to polish it with a medium grade rubbing compound such as one of the Farecla or 3M products (a fine grade might do if it's not too bad) using a foam compounding mop.
This topic crops up regularly so try a search or two and find a car paint supplier to buy what you need.
I use Farecla G3 and it's superb- costs about £22 from a car paint specialist for 2.5ltrs which last years. Don't waste your time with things like T cut of the stuff you get from chandlers as they are useless.
You need to finish with a good polish after use.
A quick way to use Farecla is to use with a damp cloth to remove the grime and then wash the residue off with a good car 'wash and wax' which also adds some protection. Not as good as a good polish but easier!
Have started to use Poliglow on the cabin top & deck. It definitely does what it says on the tin! No idea about overpainting later, all I wanted was to get the really crappy, dull GRP up to a half reasonable state. After trying various cutting products, polishes etc with varying results, none of which lasted very long, I tried Poliglow late last summer & the bits I have done are still 100%.
Thoroughly recommended, and of course the usual disclaimer, no connection with the company, other than as a satisfied customer.
The problem with cleaners is that although they will get the dirt off, they wont get rid of oxidisation.
The 30 yr old gelcoat on my boat is pretty tired and shabby so I experimented on an area by first cleaning thoroughly with yacht cleaner, then lightly sanding (yes sanding) wet with 1200 grit, then using a heavy compound designed to smooth out the 1200 grit scratches, then a lighter compund to remove the prevous compound, then a polish to finish it off, and finally a coat of UV resistant boat wax.
The result was outstanding, the area I treated is so shiny it looks like liquid, and it lasted all last season. I konw this method is a bit harsh on the gelcoat, but gelcoat is layed very thick and I think this kind of treatment is fine once or twice in a boats lifetime.
The only problem is it took me hours to do a small area by hand. I'm going to invest in a gental orbital polishing machine and attempt the whole lot before she goes back in. I'm convinced this will last for years with just a polish and wax each spring, and a mid season re-wax to keep the UVs at bay.
hi cutting compound is fine at first but remember it only removes material,the reason for using compound is the gelcoat has oxidised,it may bring back a faint shine but its short lived because compounding makes it worse by removing the gelcoat,POLIGLOW refils the exposed grp seals and rebuilds the shine GUARANTEED for more info and demos visit my website www.gelcoatrestorationservices.co.uk
This twat has searched for all threads on dull hulls and resurrected them with his crap website URL in the post.
It's about time we had some weekend moderation on here as it's getting like the sodding shopping channel..