Hi
I have some serious imperfections in my mould which will need filled.

Using Gelcoat and wax works fine in small areas but I wondered if there was some other type of filler more suitable for larger surface imperfections - like a car body filler?

I would be using the mould to make a part to make another decent mould from (I know!!)

Thanks

You could use epoxy filler to repair the damage, than take a part and polish it (because the filler won't be polishable).

Would a normal car body flexible type 'styrene' filler work - like a Upol 38 which is a two part filler but inexpensive and easy to sand?

Or would this react with the new gel coat and give me the same problems of wrinkling?

Polyester fillers usually have very bad releasing characteristics. This can damage the gelcoat of yor new counter mould, wich would bring you back were you started. Even with several coats of release agent. Epoxy fillers work much better in that case.
Wrinkles usually can be repaired quite easily with gelcoat.

Thanks so much - this is the type of information that is so useful to us beginners!

I wouldn't even bother trying to repair the current mould if the plan is to make a new one. Just fill in any substantial damage that could cause a release issue with wax or plasticene, spray PVA over the whole thing and make your copy. Put your time and effort into repairing and resurfacing that rather than the current mould, given you are going to have to resurface the copy regardless. Don't do the work twice if you don't have to.

Thanks - that had occurred to me because it will be much easier to sand off raised imperfections from the part rather than trying to infill the mould - so I think I will try to improve the mould a bit using gel coat and wax to ensure the part won’t have release issues then put the work into the new part to get a good ‘new’ mould.