OP827 wrote:I came accross this technique and thought it couldbe used to build foam teardrops...
I was thinking what if I paint scored XPS with very thin epoxy (primer maybe?) first and let it set and then laminate with polyester resin over it.
It's difficult to get poly resins to set up over epoxy. I've done it where I've scarfed plywood hull planks together and sheathed the finished hull in poly and glass, but it's a bit of a chore. The first hurdle is that the epoxy has to be cured good and hard. I made up my planks in the (heated) shed during the winter and then they sat for months until it warmed up enough to start building outside.
Then you have the blush to deal with, as it will definitely prevent the resin from bonding. Despite the advertising there are no "no blush" resins, only low-blush. ALL resins generate blush - it's a by product of the curing process. No blush means no ambient humidity, but some humidity is required for curing. Some like to sand blush off, but I've found that sanding can heat it up and smear it around. It's water-soluble, so I use a hose and a green ScotchBrite: it's quicker than sanding and you don't risk sanding any of the fibers. I'm sure proponents of sanding can offer advantages of that method as well - to each their own.
But either way, the blush has to come off if you want the poly to stick to it.
I would suggest using a coring foam like Klegecell, Divinycell, etc instead of XPS. I started on a foam/epoxy body but realized I was throwing good money after bad by using cheap foam and putting ($$) epoxy over it. I took apart some of the panels I had made and found long cracks running through the foam in places. Even though I didn't have the whole thing complete and subject to the loading it would see in use, it suggested to me that the foam didn't have anywhere near enough structural strength to work well with epoxy and glass, despite its chemical compatibility. Over time, I would expect to see exactly the same thing in the ribs and keelson of the boat shown in the video. That will allow the glass to flex and crack.
OTOH, coring foams are made for this exactly kind of loading. My next one will be another kick at the glass-and-foam cat, and I will definitely use PVC foam instead of EPS or XPS. With canvas, I don't think it would be an issue and wouldn't be worth the extra cost. Every time I bought more resin, I regretted using XPS for my coring. PVC foams are compatible with both poly and epoxy resins: they are made specifically for that job. Personally, I think using epoxy over XPS is a false economy.
I tried a few different paints and coatings but found that the coating either ate the foam to some degree or was eaten by the resin. I didn't try the floor coating (epoxy) though because I didn't want to commit to a 2-gallon kit if it didn't work...
Epoxy is the logical choice (with the caveats above) but IMHO bumps the price up enough that it becomes a wash versus using a coring foam to start with and eliminating the need for the epoxy pre-coating. Additionally, any areas that get an insufficient/incomplete epoxy coating will compromise the core and you won't know it because the glass will bridge the void but will again crack over time (as Tony mentioned).
The other problem I saw in the video was at 4:21. He split the wetting out of the fleece into more than one session. The edges of his "ribs" of resin are where I would expect to see the fibers break over time and use. You'll see that if/when you get a drop of resin on your shirt or pants: over time, the cured "drop" will break away, leaving a hole the shape of the drop. He obviously did go over it all with resin, but that's a purely mechanical (rather than chemical) bond and will concentrate forces along the line of the bond. The best option there is to glove up, turn your phone off, and glass the whole thing in one session. That would also make the mat he put on the inside redundant. The small amount of "oil canning" at 6:46 doesn't warrant the extra weight (and $$) IMHO
There are other issues, like how precoating the foam with epoxy greatly reduces the surface area for the poly to bond with by 'smoothing' the foam. It seems this is a deliberate introduction of a weak point for the sake of saving a small amount over the course of a build. I'm sure others will agree, the cost of skinning is a small percentage of the overall cost of the build but YMMV.
I also tried polyester resin (lumpy, not as good impregnation, rough surface, etc.)
It sounds like you didn't roll it out ?
Looking at the book I just wrote, I'll shut up now...
JMHO and all that.
Any questions , just ask !!! 
