Got a carbon fibre bike off ebay, in excellent condition apart from some marks on the rear dropouts caused by putting it in a turbo trainer.

What is the best stuff to use to cover it and seal it up?

I used nail enamel on my carbon XTC frame to seal chips and scratches in the lacquer.

pick up some water based wooden floor varnish. Choose a satin or gloss depending on what you have.

Then 2 approaches:

if you think you can just touch up the local area then remove and rough edges/loose material with some fine wet and dry or if not gentle rub down the whole section. Only use something like P1200.

Then carefully brush it on (airbrush spray is better if you have one) and let it dry for at least 24 hours away from any dust. Don’t expect a perfect finish though – although you can always rub it down with wet P1200 a few days later – but only gloss.

Or if they are just fine scratches then just try rubbing them with wet and soapy P1200 wet and dry or just leave them as it’s a bike and going to get marked anyway!

edit: Halfords do a spray PU lacquer for alloy wheels which will give a better finish. Wear a mask when you spray it.

The best stuff would be epoxy resin. Not necessarily the cheapest.

http://www.easycomposites.co.uk/products/black-epoxy-repair-filler-for-carbon-fibre.aspx

The best stuff..

Nope. Maybe if you have a hole in something black that you want to do a cosmetic fill on. But not for surface marks.

You can get stuff for repairing carbon fishing rods that may do a job other than that i would get proper epoxy resin

You can also get a graphite spray that may cover the imperfection