Doug,
Below is a link to Frappr site that has some fabrication pics of my boat. I hope this works. http://www.frappr.com/?a=viewphoto&id=2664457&pid=4280217&myphotos=1
The procedure I used to mix the resin with the microspheres was to first calculate the weight of each constituents; microspheres, resin part A and B, so that I would end up with about 4 gals in a 5 gal pail. I then zeroed out the scale with an empty 5 gal pail sitting on it. I then added the resin parts A and part B and mixed with a paint mixer attached to a drill for 1 minute. To this I slowly added in the correct weight of microspheres while continuing to mix.
You have to be careful not to overmix as this can cause breakage of the microspheres. At this point the solution has the consistency of pancake syrup.
Prior to mixing the resin/micropshres, I filled up the fibrgalss compartment completely with macrospheres through 1" diameter holes located longitudianlly along the top of the boat. I made screen mesh inserts for the pour holes to keep the macrospheres from floating out of the pour
holes when the liquid is poured.
I then poured this brew into the fiberglass compartment through 1" diameter holes. I contnued this process until I filled up the intire compartment. After about 1 hour you could feel the temperature spiking along the side of the fiberglass compartment. After 1 day, the entire casting was rock hard.
The final resin % was determined from bench experiments. Resin is the heaviest component of the foam so you want to minimize this volume. But if you do not have enough resin, when you add the microspheres, your solution becomes to thick having the consistency of peanut butter and will not pour. If you add to much resin, the microspheres tend to stratify towards the top of the casting and the excess resin moves to the bottom. On the frappr site I listed, I have pics of two of the bench castings one of which shows this stratification phenomena.
As to using
a vacuum, If you can rig this up, it would help to remove air from the resin/microspheres solution before pouring. In my case, I did not need to do this as test pours confirmed that I could place the foam as described above and was getting castings without any air void spaces.
As to the question of epoxy resin versus polyester resin, a commercial manufacturer of syntactic foam told me that you could use both but that epoxy would give a stronger and harder casting. In my case, I was using the foam to strutually support my two ton boat while it sits on the trailer so I need something that was not only low density but struturally strong enough to support the weight. I think it would depend on the design depth at well. For boats designed for deep service I would use epoxy but for shallower design depths, my guess is that a polyester resin would work just fine and would be more cost effective.
My experience is based on this one boat casting
so I would suggest talking with others that have broader experience to nail down this issue of epoxy resin versus polyester resin for your application.
Cliff