I have used polyester for non critical items, above waterline.  
You need a rough surface to the timber to get a good bond (planed is a disaster).

prime with resin thinned with styrene. Continue whilst still tacky. 

always add some fibre / filler to the resin, this gives it more flexibility. Treecell is good. 
i found that vinyl ester is not much cheaper than epoxy. 
for epoxy try Epodex, it is a very good price. Made primarily for work tops, so is very thin and slow setting, so soaked well into timber. 

 In my opinion I prefer ringed nails in plywood over screws. 

Bronze ring nails available in the UK here;

https://www.seaware.co.uk/products/gripfast-nails-silicon-bronze

Hi, I’m strengthening the area around where the front hatch used to be on my Hurley 22, ready for the mast. I started with a 9 mm ply sheet that spans most of the deck, then added a 25 mm ply sheet covering about 70% of that (all exterior grade). I’ll build up more strength around the mast with smaller layers of 25 mm, using Mingming II as a guide.

I know epoxy would have been ideal, but I’ve used vinyl ester as I’ve got a lot of work to do on the boat and didn’t want to juggle both epoxy and polyester resins. Vinyl ester is still waterproof and reasonably elastic, from what I understand.

My question: I’d like to add extra strength between the layers with some kind of fastener, as Roger Taylor did, but I’m not sure what to use.

I tried self-tapping 316 screws, but they strip out easily (maybe I just need to pre-drill). Since it’s plywood, should I use washers or screw cups? Or would brass nails be better? Or am I overthinking it?

All the ply will be glassed over in the end. There will also be two wooden blocks on either side of the mast for the turning blocks, which will be bolted right through the deck — but I’d still like a bit of extra strength around the edges of the reinforcing area.

Thanks for any advice!