Helmet Help !

So I just started my first mando armor build. So far I have my helmet almost done the things needing finishing are the dome and visor, for the dome it is a cardboard dome capped with dry wall spackle....is there a way to harden the spackle such as using PVA glue ? (planning on sealing it all with Polycrylic sealer) Attached is a pic for reference
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You might get more help if this was in the helmet section. If it's that "one-time" soft vinyl spackle, it may never harden as much as you like, but sealer would work. Use it thinned for the first coat or two and it might soak in more, but give it lots of dry time.