Polyester resin will dissolve polyurethane foam. It will make a gooey mess. Also, the labor involved in doing something like that will more than offset the cost of proper foam. You could get a job at minimum wage, pay for structural foam and still have leftover for beer.
You're right in that it was labor intensive to do that. I mostly did it to see if it would work, with a subtext of being extremely cheap as a result of lack of money, and large dislike of paying 8.5 times what something is worth just because it is labeled "marine".
What I used was 3/4" foil faced, tan colored foam from Home Depot. Technically, it was polyisocyanurate foam, a form of polyurethane foam. From what I can tell on the internet, polyester won't melt polyurathane. I know for a fact it won't melt 'polyiso'.
The foil faced foam was very light and friable, easily crushed and a lamination of plain csm easily came apart as the glass stuck to the foil but the foil peeled right off the foam.
I tried it different ways, threading through from one side to the other and then back through etc. That was a nuisance because you had to pull the whole "thread" (roving) through for each stitch, which broke down the roving and was very labor intensive.
Then I tried using short pieces that I just poked halfway through, which left a short end on each side. Next I tried short pieces where I poked the middle through, which left a loop on one side and two ends on the other.
That led the way to the last solution which is what 3M does. I just layed a string of roving on the foam and poked it through every so often, leaving a line of flat threads on one side and a line of loops on the other. In 3M's photo, The lines with flat threads have loops on the other side and the lines of loops have flat threads on the other side.
Once I got to the last technique, I made an 8' x 8' panel for my workshop that could be easily removed to bring big stuff in and out. I made a light frame of 3/4" x 1" sticks around the edge and then divided that in the middle horizontally and vertically so I had four 4' squares. The top half I divided up some more into 'windows'. I fitted in my stitched foam into all the spaces but the windows and then vacuum bagged it all with poly and 1 layer of 1 1/2 oz mat on each side. I ended up with a Japanese Paper Wall type of thing that is very light weight and has been in place for almost 20 years. I could easily bust through it if I took a run at it, but it has stood up to the wind and rain for that long and I only put stitched loops spaced every 6", as in 1 sq. foot would have 8 loops around the side and 1 in the center. I imagine with as much stitching as the 3M photo it would be very strong.
I also used the Polyiso foam to build a jonboat similar to how "Carolina Skiffs" build theirs.
http://www.polyiso.org/
It's a very common commercial product that can usually be gotten at wholesale insulation prices, which is like half the price Home Depot would ever charge and 1/10 or 1/20th the price of "marine". They make it with or without the perforated paper face and in all sorts of weights and thicknesses. It's used a lot as the base for tarred roofs, so it comes in structural densities that can be walked upon by construction crews or used in SIPs, structurally insulated panels.
As for working at minimum wage to buy structural marine foam being a good way to do it, it depends on what you need it for, how you go about it and how much it would cost. How much does a 4x8' sheet of 3/4" structural marine foam cost now? I have no problem at all with other people spending their money for expensive items, but if it's me, I start thinking of alternatives.
I was looking at the foam in the 3M photo and thinking it looked like that flexible packing foam and sure enough, I think it is.
http://multimedia.3m.com/mws/mediaw...vTSevTSeSSSSSS--&fn=3M RTM Core Data Page.pdf
The stuff sounds pretty useful. Being flexible it can bend to compound curves and still have 250% more rigidity and 1000% more resistance to shear failure than other materials, and the prices even seem reasonable to me.
See now, I immediately think of that cheap flexible foam that goes under laminated wood flooring and that with a little thought it wouldn't be hard to automate or semi-automate a way to stitch in roving as simply as 3M's is stitched in.
It looks like the one guy could use it in his go kart bodies for a vast improvement and Frosty could try making a BBQ out of it.
:?::idea:
