+x Hanaldo - 7/1/2018 2:05:11 AMI wouldn't sand it at all if its plastic. Just use a blade to deburr the hole and then fill with wax.Just a side note - be aware that if you are using a polyester or vinyl ester gelcoat to make the mould, then these will very often attack uncoated plastics, ruining both your original and your mould! So I would advise doing a test in an unseen area, and if in any doubt about compatibility then using PVA release agent to protect it, and later refinishing your mould to a high standard. You may also want to support your original so that any heat from the mould curing won't cause the original to warp and leave you with a distorted mould and parts that don't fit.