Depending on the type of "canvas" it may work just fine ..   It will add some strength, but not nearly as much as glass-fibre..

What you should have bought at the local hardware store is  "Wall -glass mat"  of the cheapest type without any primer or binder in ..

Works a treat  with Epoxy ... and its cheap ..  No idea how it would work with Polystyrene though- --

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

Well what's the modulus of canvas? Denim is a cotton fabric and will absorb a lot of resin but it doesn't have any strength. Don't forget that resin itself is very weak, it's the fibreglass that makes things strong/stiff. 

It's a heavy cloth that saturates with resin...any reason why it wouldn't work?

Canvas? Interesting. Haven't heard of that being used in composites before? 

Thanks for all the help. I took your advice and applied two more layers of heavy CSM then heavy weight canvas I bought at the local hardware store.

A lot of effort to go through for little reason. Firstly, the first layer behind the gelcoat is the critical one that you don't want voids behind. The gelcoat is what needs the support, after that first layer any voids in the mould aren't too critical. Obviously this is within reason, you don't want large areas of bridging or large/lots of voids. But regular laminating techniques, so good stippling with the brush and then going over the entire mould with a roller, will ensure that those voids don't occur. 

Thanks.

Looking good. 

Sanded the whole mold down and filled some voids with Duraglas...

I used a slurry of chopped glass fibers mixed with resin and worked it into the corners all the way around.