Complete newbies to making anything from composite materials so just after the most basic information and material recommendations if possible.

We're looking to make a seat base for a bike we're modifying using fibreglass/composite and would like some assistance choosing the materials and gels, resins etc.

I realise we'll need a release agent or wax but to make a strong seat base that we could drill and mount to the frame, what sheet material would be best and with what resin?  It will never been seen so we're not interested in the finish per se rather the strength.

I've seen a video where some wraps or lays plastic sheeting over the area to be glassed over and then just lays the sheet material over the top; is there a particular type of resin which is best for this and won't attack the polythene sheeting.

Apologies for the rather woolly information but like I say, this is an entirely new venture.

Thanks, Ed

If I am understanding your question correctly I would probably purchase a G10 tube of the appropriate diameter and wall thickness which I then epoxied the seat I made onto with a nice big fillet (made of epoxy thickened with mill fibers and microspheres) which was held in place by a couple layers of 6 oz biaxial fiberglass tape.
(disclaimer: all my personal experience is with epoxy vs polyster/vinylester which factors into my recommendation. I know that epoxy doesn't stick well to polyethylene and it will not attack it either. The reason I suggest simply purchasing a G10 tube is that it will let you achieve a much higher level of precision and strength where it matters without as much technical ability. Also it's worth choosing 2 of 3 features when thinking about composites: lighter, stronger, or cheaper.. For example you could do carbon fiber seat post instead of G10 to reduce weight but spend more money for a given strength..).
Best regards,
Dave
(P.S. I doubt we're in the same region of the world so I didn't see any value in recommending any specific product or suppliers, but for your application the main consideration for epoxy is the curing speed and ideally choose a cycloaliphatic non-blushing epoxy if going the epoxy route, and for fiber reinforcement E glass is probably just fine for you but if you want to spend more to save weight without compromising strength you can go with S glass or basalt fiber..)

Thanks for the reply.

It's not a seat post I'm looking to make but a seat pan which we can then add some foam layers too before upholstering.  We've removed the metal seat pan from an '82 Honda and want to shorten the seat unit to give it a more aggressive look.  Due to the various lumps and bumps of the old seat mounting we have considered using fibreglass (or similar) to incorporate these rather than making too many frame alterations.  Our thinking is that we can then drill and bolt this new pan directly to the existing mounting points.

Sorry if I didn't make that very clear in my original post, the picture below might help.
Regards, Ed