Chemical compatability question - Polyetser and polyurethane resins.

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Rick1138

Member
Hi folks,

I am in the middle of a cosplay build and I am working on hardening my helmet piece.

I have stiffened the cardstock with brushed on casting resin, however I am about to do the proper stiffening stage inside the helmet and I wondered if anyone could confirm something before I proceed.

basically I have 2 options, do I use Rondo type material to slush cast the helmet onto the resin coated paper.

or do it slush cast more resin into the helmet making a solid plastic helmet.

This helmet is going to be a master mould for making a silicone jacket mould.

Basically can I lay polyester resin\bondo\glass fibre onto cured polyurethane resin\plastic. Or will it react and forma bubbling mess o doom?

Any help would be great as googling or searching on here is causing all kinds of probblems due to the similarities of the words, resin, casting, cured and reaction.

Any advice is welcome :)
 

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Kevin Gossett

Master Member
RPF PREMIUM MEMBER
Lay your fiberglass mat/cloth on the inside using the same resin you used on the outside. Then you can bondo the outside for smoothing and details
 

coldfire

New Member
i dont see why it would react but, if it is just a master for molding I just do a coat of resin on the inside without fiber glass then i will melt some hot glue and brush it on the inside until it is a 1/4 inch thick then spray foam the inside it give enough strength to mold from. especially after you bondo the outside. this has worked for me and it saves a lot of time
 

Rick1138

Member
The only issue with this is that fibreglass matting I have doesn't work with casting resin, it needs polyester resin to melt the fibres. Epoxy resin may work but I don't have any of that. I do have normal polyester resin (Bondo UK equivalent) but I am worried about using it on the cured casting resin coat.
 

YenChih Lin

Sr Member
CHemist here, PU Resin is one that cures with an exothermic reaction, means, it releases heat. If you want them seperated, don't directly cast them in.
 

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cavx

Master Member
RPF PREMIUM MEMBER
Just my $0.02 worth. If you started this project with polyurethane, then stay with that. Don't add polyester into the mix now. From my limited experiences, it will just end badly.
 

Duncanator

Sr Member
If the urethane resin is cured there shouldn't be any problem. You should be able to use the polyester resin on it just fine. FYI- Bondo is a filled polyester resin and works great on urethane.
Since you are planning on molding your pattern, it only really needs to be strong enough to hold its shape. It doesn't have to be a tank. I've always said with silicone you could sculpt your pattern out of butter and get a good mold. (Ok, a hot molding day would scupper that, but you get my drift.)
 

cavx

Master Member
RPF PREMIUM MEMBER
Since you are planning on molding your pattern, it only really needs to be strong enough to hold its shape. It doesn't have to be a tank. I've always said with silicone you could sculpt your pattern out of butter and get a good mold. (Ok, a hot molding day would scupper that, but you get my drift.)
Agreed. I recently was altering a sculpt just prior to taking a mold and used Plasticine as the build up medium. It worked a treat and was easy to remove later.
 

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