I've got to replace some port light and I've been given some nice aluminium opening jobs that will clean up good, however I will have to make up a "flange" for them as the cabin side a 50mm and opening in is the only option. So the plan is to make an internal mould out of wood then epoxy glass tape around the mould, then when the epoxy goes off, pop the flange of the mould and repeat. Suggestions for a release agent would be appreciated

Bees wax will get you out of trouble, cooking spray too but you will need to be sure the timber mold is lined with a plastic and very carefully too. Spray onto epoxy side of the plastic. Timber is porous and can prove troublesome to release from. Also keep in mind the mold is going transfer in detail to the finished product.

Packing tape is as good as any.

Some common types of plastic film won't stick to epoxy such as a disposable drop sheet, or other LDPE film.

I do a fair bit of epoxy moulding. I like beeswax too. It seems to work well.

BUt also I like to use a trick for shapes with no taper. I often make a part out of just one or two layers of 4 oz glass and then cut through one section of the part to get it to release. Then reglue it on the bench and go adding more glass from there - even popping it back in the original mould after giving it a little sand to make it ever so slightly smaller. I have had some absolutely brutal times smashing moulds and parts apart if there is no taper. So the above trick helps me a lot.

Thanks for the input. I use duct tape between bits I don't want stick to the glueing bench it does release but sometimes it takes a bit of effort to let it go, and that's just a glue line, packing tape could be better. There's no need for a taper on the part, I was going to incorporate a small channel for drainage so perhaps a bevel on the bottom edge would be better. So far it looks like packing tape, bees wax & a modified plug and a cut if all else fails. Thanks

That was an interesting episode though a bit time consuming. The packing tape and wax both work well as epoxy non stick, the tape not so flexible for odd shapes and the melted wax paints on good, but it does add another layer of tasks it will need to be removed from the glass before painting. So back to the tried and tested, glue up ply and epoxy glass over it

Thanks for coming back with results, Woko.