Suitable material for one-off GRP mould?

[TQA]

In a similar situation I have used blue foam [ small cell urethane foam ] Hot wire cutter or electric bread knife. Glued with spray adhesive [ check to see if it dissolves first ] or epoxy. You can sometimes get offcuts from house builders or go to Sheffield Insulation.

It sands easily. P38 or spackle will fill any holes.

Depending on the shape you can cover with parcel tape to get a good release.

Worked for me.

 

[blxm]

To be honest you can use almost anything. I have made lots of positive and negative moulds over the years. it needs to:
a: be strong enough to withstand the laying up process (no need to be that robust)
b: have a surface finish acceptable for the final product (gel coat is very hard! you don't want to be polishing out surface defects)
c: releasable from the finished product

I have used everything already mentions to make the mould. If you cant get a taper (draft) on the mould just make it in two pieces, more difficult from a rigidity point but not impossible. for releasing agent you can get paint on water soluble release agent good for one offs, car wax and silicone grease work (just). Cling film (thin plastic) works a treat but can be tricky to get a wrinkle free finish. some plastics (like the Jacobs biscuit box (other biscuits are available) in a previous post) are excellent and probably wont even need a release agent as the gel coat wont stick to it. For multiple releases my preferred option it honey wax, there are other brands available(!) that you can get from fibreglass suppliers. Loads of coats to start with then just reapply a couple of coats per lift. I have one mould that is approaching the 100th lift using this stuff.

It not difficult, have fun and create some interesting shapes.

 

[gjgm]

Thanks all again-given me confidence that this can be done!
Being wet and wintery, this may be a spring project...
Sorry to drop into OP thread...

 

[salar]

All the moldings in the wheelhouse were made using one-off molds consisting of melamine faced board and Plasticene. Here are some pictures, others are on the blog http://harley25refit.blogspot.co.uk/ : helm side panel, instrument pod, complete galley, seat bases and in the cockpit, steps and a locker box.

 

[Avocet]

Just a thought, but as an alternative to any kind of mould, I have made fibreglass "box shapes" without any kind of mould, simply by cutting flat sheets of fibreglass out and then "taping" the edges together with fibreglass strip. You can use a hot glue gun to hold them together temporarily on the face opposite the one that's being taped, then scrape the hot glue off afterwards.