Identifying epoxy or polyester

[David P Williams]

I have a large fibreglass cowling moulding that I want to modify. Is there an easy way to identify whether it's polyester or epoxy? I understand that they don't play well together.

Is it easier to bodge it to how I want it then make a new mould and cast a new epoxy one? Any gotchas to watch out for?

Thanks,

David

[Alan Gorham_]

Sand the inside with some coarse glasspaper, say 120 or 180 grade. The smell of polyester will be very strong and "plasticky" - just like polyester resin. Epoxy frequently has a very faint smell or none at all.

 

It's hard to comment on your proposed modifications without knowing what they are, but GRP moulded (regardless of the type of resin used) are simple to reshape, rework or repair.

[David P Williams]

Thanks Alan. I need to shorten the cowling, but of course it tapers, different angles all round. If I am reshaping or reworking it, can I use epoxy regardless of what it’s made from or must it be epoxy on epoxy, polyester on polyester? 

[flight1]

If in doubt use epoxy as it will stick to itself or polyester will be lighter and stronger, just prep properly(degrease and roughen to key up)

[Alan Hilton]

If it’s glass chop it’s got to be polyester ,epoxy won’t work with chop due to silicone which is used to hold the chop together . If it’s glass mat it could be ether so smell it 

[Alan Gorham_]

That's not strictly true, powder bound chopped strand mat is perfectly compatible with epoxy resin.

[Paul De Tourtoulon]

Polyester dust smells like marzipan,,,

 

 Don't forget Epoxy sticks to Polyester but Polyester won't stick to Epoxy.

[Alan Hilton]

2 hours ago, Alan Gorham_ said:

That's not strictly true, powder bound chopped strand mat is perfectly compatible with epoxy resin.

Haven’t come across powder bonded chop is it common ?

[Engine Doctor]

Just stick to epoxy for epoxy and polyester for polyester then you won't have to worry about silicone or powder bound chopped mat. With either type , clean ,degrease and abrade thoroughly any areas to be bonded prior to starting work.

[Andy48]

Use brown Gorilla glue, it works well with both epoxy and polyester provided you lightly sand the bits to be glued to provide a good key and get rid of any moulding release agents. It does foam so the pieces need clamping together.

[David P Williams]

Thanks everyone for your helpful replies.

This is the problem I have (serves me right for wanting a totally different engine than the kit was made for) I'm trying to leave the cylinders exposed and either just cover the exhaust collector ring or butt up to it with a small gap

 

 

and this is the inside of the moulding. I've tried sanding it and it maybe smells a bit marzipan-y but my sense of smell isn't great

 

 

The more I look at it and try to measure in three dimensions, the more I think I need to start from scratch and make a mould somehow. It's supposed to look something like this. I obviously won't get true scale representation but a reasonable looky-likey will do.

 

 

[Piers Bowlan]

Epoxy/glass by the look of it.

[Paul De Tourtoulon]

No, it's chopped strand mat = polyester,,,,

 

 Woven mat = epoxy,,,

[flight1]

2 hours ago, Piers Bowlan said:

Epoxy/glass by the look of it.

? your observation is good

oops bit sarcastic me?

 

Well it's made from chopped strand glass and a resin of some sort which you can not determine by a picture so with alterations use epoxy and woven cloth it will work good.  the fact that it is chopped matting doesn't matter as it has cured .

also with matting  you can get Emulsion bound  for Polyester and Vinylester resins and powder bound for epoxy so its all a guess unless you can ask the one that made it

 

I would use the back end of your existing cowl cut off where the yellow stops on the picture of the real one and then with the front bit you have cut off to simulate the manifold/exhaust cover on the real one . then laminate the bits together together.

Edited December 7, 2021 by flight1
more info

[Allan Bennett]

I would be sorely tempted to use that cowl as a master to make a mould from.  Add all the little bumps and other raised details using P38 or equivalent, cut or file the panel lines and other recessed details, then make a mould of it.

[David P Williams]

Yes I think hacking the cowl around and patching it up is the first go, and then possibly using it to make a mould. I need to make it a two piece affair anyway or I won’t be able to fit it.

Morris at Morris Mini Motors has confirmed I can cut down the exhaust headers by 15mm to move the collector ring closer to the cylinders for a more scale look and to give me more wriggle room.

[flight1]

that's probably the best way to go- hack the cowl until you got what you want the use it as a plug to make a mould for a new one. lighter stronger and a better finish.

 

made a custom petrol fuel tank that way recently, took the same time as trolling through the internet trying to find one that fitted my application.

[Paul De Tourtoulon]

If you have the know how, go with making a mould and moulding a new epoxy / roving one, it's lighter and stronger than Polyester.

 

 you can always make a plaster of paris mould as it's for a one off,,,

Edited December 8, 2021 by Paul De Tourtoulon
plaster

[David P Williams]

Now I’ve done more measuring and faffing with cardboard templates I think a cut’n’shut might do the trick. 

[Christopher Wolfe]

I would pop a mould (plaster ok) from the cowling before butchering it as then you have a starting place.

 

Also a plug with holes in it can present few release issues.

 

chris

 

[David P Williams]

Thanks Chris, good tip