poly/epoxy questions

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#1

I’m new to plug making but i’ve been a GRP repairer in the marine industry for 7 years.
My first plug, is a bit of a big project tbh for my first plug, it is a car door. I’ve been using all polyester composites as this is what I know. I’ve made the plug now, it has turned out ok, but it was slightly difficult to release, but no damage to it so fairly happy. My plan is to make the final door in epoxy, But I have some questions.

  1. Have I made a mistake making my plug from polyester? I keep reading about shrinking issues and I can’t really have this with a door.

  2. Will I have problems with a poly plug and an epoxy mould?

  3. Is epoxy compatible poly gel coat any good? Is this the best route for me as I will be painting the final door.

  4. For plugs that I am not overly bothered about with the finish, is there a cheap suitable solution rather than spraying and rubbing down as the expenses add up doing it this way. I’ve seen a guy on youtube using a varnish,is this really a suitable way? I would like to brush on to if poss. What have you guys used?

#2

Nothing wrong with polyester plugs, make sure it gets a two part paint. Sure polyester shrinks a bit but as long as the overall job is well supported with strong a base or other then there’s no problem.

Plus use a sealer and mould release system, semi permanent or wax (or both) on the plug.

Make a polyester mould too. Why use epoxy?? An epoxy mould is expensive so I would not do this unless there is a specific reason to do so. Epoxy means, more expensive resins, more expensive fabrics and more labour to layup as putting down layers of cloth is far slower than getting thickness with chop strand matt.

Doesn’t make sense that you are concerned about the labour in getting a nice plug finish yet wish to make an epoxy mould??

If you really want to avoid spray equipment for the plug finish then use shellac and apply with rag as with French polishing. A shellac finish will only need wax for release.

#3

The reason I was thinking of epoxy, is I got hold of a 20kg tin of it from a shop that closed down next to my work for next to nothing and I was thinking epoxy would be better with the stress of a door flexing. With my plugs I am only concerned about the finish of the skin, the inside part I’m not fussed about the finish. Will a polyester door have fitting problems though if it shrinks??
can you give me a link for shellac? I’m not quite sure what it is tbh :unsure:

#4

You can use epoxy for the mould, I have only made a couple of moulds using it and they were expensive, I used an aluminium filled gel coat which cost a bit and then the cost of using fabrics is far more than chop strand plus the extra labour to layup compared to poly.

I would consider saving the epoxy for your parts since I always shudder at the thought of stinky polyester inside your car.

Shellac is cool, it’s a dried beetle excrement that comes as brown flakes and you need to dissolve it well in methylated spirits. This is what was often used in old school fglass shops for patterns. I have found that it only needs three coats of wax and that’s it. Nothing sticks and even if it did apparently the shellac surface will just tear apart which means no damage to your pattern or mould. Then any shellac still stuck to your mould will just dissolve again with methylated sprits so even a stick up job is a still a success! It can seem labour intensive to apply but it dries instantly so you can keep on applying coats one after the other. Typically you add more methylated spirits as you progress and this thinner mix makes a smoother finish.

http://en.wikipedia.org/wiki/Shellac

Using shellac may take a little getting used to but I found great and safe. Its quite hard and sands nice in-between coats but strangely makes very little dust when you sand it.

There are lots of how to videos on YouTube but much of this relates to finishing furniture. All the same but maybe a plug does not necessarily need a piano finish??

My pattern routine is still finishing with sprayed two pack paints but I like to finish any temp flanges with shellac coated MDF board rather than melamine board. I found the melamine just releases too easy and I would often get pre release on my flanges during the gel coating or skinning layer.

#5

Very interesting, I had never heard of this before. Sounds like it could also be good for complicated shaped where releasing could be a problem normally. What was the advantage of using a gel coat with aluminium in it? Was this because of using epoxy?

#6

I was hoping that the aluminium filled gel coat would make for a more durable and chip resistant mould surface for use with pre pregs. Unfortunately it still chipped (on sharp corners)so I now persist with good quality tooling gels and just improve my techniques instead.