Rebody Fibreglassing help

[Pond]

I am going to have to do a bit of fibreglassing as I can't avoid it!
My questions are:
For very small areas, ie 5mm x 600mm x 10mm thick can I use pre-mixed fibreglass filler? Will it be strong enough?
I want to make very small makeshift 'moulds'. Do I need a specific release agent for the mould or will any wax do? I have carnauba and some beeswax-based stuff for furniture. Any good?
Will I need gelcoat, or will just matting and resin release from a mould OK?

 

[Duncodin]

5mm x 600mm ?

That's like a narrow slot 2 feet long. Joining two panels? Or are you adding to the edge of a panel.

For 5mm I'd use the filler with fibre in it like isopon P40.

But I don't just stick it on. I really chop into the edge of the panel to rough it up. A lot.

I don't use gelcoat. I get the grp filler just below where I want the surface to be and skim with smooth filler. Something like P38. Then filler primer.

 

[Pond]

Thanks Ian.
I am hoping my paintshop will do these bits for me, just ascertaining what is required if they can't (or won't).