Thank you again for your reply.  Yes it is a card tube:



The diameter is 295mm.  My intention is to build it up to 300mm with woven glass (for various reasons). then prime it with pattern coat primer and high gloss then polish and create a female mould with flanges running down both sides.  If I am using woven glass do I still need the primer and high gloss?  Oekmont, is there a better way of making the mould, sorry I didn't quite understand your description?

In my opinion this is far to complicated and unnecessary expensive. And will likely result in an uneven surface. It is quite difficult to sand a tube perfectly round without a jig.
I would take a kg type tube (used in plumbing) wich you can get at 300mm diameter and 1,5m length. The finish isn't that great, but it's straight and even. Wich will be hard to achieve with your method. Then take the mold, remove the tube, trim the edges and close the mould again. In this closed state, start sanding the mould beginning with 400grit. This ensures, that your flange edges stay sharp. Then use high grits. 800, 1000, 1500 and finally 2000. Finally take the mould apart and polish it up to high gloss. This will give you a nice finish.

Sorry oekmont I am completely baffled.

You want to take a negative mould from a 295mm cardboard tube, wich you want to cover with glass fibre until it's 300mm diameter, right? To finally make a carbon tube, right. I would not use an enlarged cardboard tube, but a tube, wich you can get a better hardware stores for cheap money instead. Because it is plastic and hard to polish up nicely, I would instead take the mould from the raw tube, and sand and polish the mould afterwards. Because the surface is already quite good, that would not take long, and the should be no danger to sand through the tooling coat. In the end you will safe much money and even more time. And you'll get a a better finish.

I would indeed have used a plastic tube for cheap money if I could get one where I live.  Unfortunately all I can get hold of is a 3m length of Sonotube, however it is incredibly straight and round.  I think I follow you now, making a mould and then finishing it post cure, rather than polishing up the plug. Good idea.  It is difficult for me to follow sometimes as this is my first project, and I am therefore erring on the safe side of assuming the plug has to finished to a very high standard etc.  Thanks for your input, it is appreciated.

You can even get 300mm tubing at amazon. In Germany you have to search for "kg rohr dn300". Every good hardware store should get their hands on that dimension. This is used in every bigger building.
If you do the plug with primer etc, you should definitely try to get a good finish on the plug. There is too much danger of sanding through the gelcoat.