If you were to take two equally shaped eps blanks and layup one with a squegee and vac bag the other which one would be more durable? Please explain?
I’ll be interested in hearing feedback from others with more experience than I, but the way I understand it, vacuum bagging allows you to build a lighter overall layup. Thus for the same weight, you will yield a layup that has a higher glass to resin ratio which will be stronger than a layup that has more resin and less glass. At least that’s what the boat builders I’ve talked to told me and is one of the reasons why I use it. The other reason is that the vacuum bag does a much nicer job of laying glass cloth around diffiicult to work areas than I ever could do without it.
some guys who really know their stuff here end up with an almost finished product right out of the bag.
No additional coating required. just a little rail scraping and buffing and they’re done.
I believe RDM with autoclaving seems to be the method to use with vacuum forming for the best strength to weight ratios from all the composite articles I’ve seen, but that’s alot more complicated than bagging a board hand glassed on both sides to squeeze out the excess resin into a blotter.
I don’t know how pre-preg factors in but that’s seems easier than hand lamming then bagging it.
Just unroll out your glass pre-infused epoxy from the freezer on both sides then bag and put it in a heated box to activate the epoxy cure and let it bake to as hot as the core will withstand.
The only benefit I can see is if you can put on more glass but tighten the glass to resin ratios by bagging it to keep the weight down. That way your skin would be thicker but not as heavy as a hand lam version.
I believe that bagged on offers the possibility of a better glass to resin ration which makes for a stonger, lighter, more flexible glass job. This is the reason airplanes are made this way. However, in the real surfboard world I doubt that it really makes too much difference. Bagging does end up costing more because of the consumeables, and it offers the possiblility of major goof ups. I just finished ripping the glass of a 9’6 that I was bagging on. I was trying a new technique and things didn’t go so well 
I’ve tried many different techniques (I think I’m up to 5) and each has its own strengths and weaknesses.
If done properly a post cured vac bag, or autoclaved product will be FAR superior, next is pre-preg fabrics (glass or carbon sold with resin already impregnated into the fabric, you just lay it down and it air cures - also needs post curing) and least reliable is hand lay up.
The reason is that the strength of any laminate comes from load being passed through the resin onto the fabric - resin doesnt deal with load very well but is excellent at holding the fibres in their place, fibre is excellent at distributing load but needs resin to help it keep its shape.
The closer the ratio of resin to fibre, the more efficient the load transfer and the stronger the over all matrix.
Often you will see surfboards with hairline cracks at various load points - this is where the resin has failed to transfer load to the fabric and cracks - ususally because there is too much resin involved, or a delamination that causes the fibre to flex more that the resin is capable. A perfect ratio laminate will flex just enough to distribute the load/impac forces without cracking/delaminating the resin - both resin and fibre at their limit but not failing.
In fact, the whole hotcoat/gloss coat process actually makes boards weaker because you are adding more resin to the laminate and therefore upsetting the load spread efficiancy. But it looks a lot better than a laminated weave in most peoples opinion, so it continues to be standard practice…
different resins distribute load in different ways hence some are better at it (“stronger”) than others. Expoxy creats long molecular chains when curing where as polyester resin makes shorter ones but they line up more uniformly - vynal esther makes medium ones that look like litte spiderwebs without the concentric circles radiating out from centre.
hope that helps,
Riff
Besides the extra cost of consumables, there’s also a lot of waste (resin) that is drawn from the laminate and thrown away. I’d guess that more than half of the epoxy I spread out on a laminate ends up in the breather cloth and peel ply. The upside to this waste is that I’m assured that I have a fully saturated laminate, but with very little excess epoxy. I’ve never tried pre-preg fabric, although that should eliminate/or reduce the epoxy waste issue.
I don’t have any specific data to prove this, but I personally believe that vacuum bagging makes a big difference in final board weight (although that may be partly due to my amateurish hand laminating skills). My last 3 boards were all done with a bag and the final product came out a lot better than I had hoped for. If you’ve never tried vacuum bagging, I really think it’s worth giving a spin. For a vacuum pump, I’m using a retired AC evacuating pump that I bought from an automotive AC repair shop for $20 and all my consumables are from a local fiberglass shop (Fiberglass Hawaii). Incidently, all my boards were constructed with either D-cell or Airex foam, S-glass and WEST epoxy. I usually do a board in two runs, first the bottom, then the deck. This takes twice as long an wastes twice as much materials, but is pretty easy and fool proof. Not that I haven’t made mistakes, but after the first board I learned to look for “problems” with the layup while it’s under vacuum and I’m careful to fix those issues before the epoxy sets up. I also use the slowest catalyst I can find to give more time to look and fix. Keep in mind, I’m in Hawaii where the average temps hover around 80 degrees F.
From my experience bagging, if done correctly will produce a stronger lighter board. However it is more complicated than a hand lay-up and increases the cost of the board. Also the relatively poor mechanical properties of the foam that make up a surfboard are almost contradictory to the purpose of vacuuming in the first place. The vacuumed cloth and resin mix will be stronger and lighter but the core will still be very prone to compression and the resulting delamination will cause the board to fail.
Remember the bagged lay-up if done properly should produce a lighter and more flexible matrix of glass and resin. Having a higher resin to cloth ratio will be heavier but will also be less flexible. The lack of flexibility is what can cause failures. In the case of a hand lay-up this extra rigidity may be all that is preventing you from causing the foam to compress and delaminate from the glass. If you were to insert a layer under the glass that was resistant to compression then bagging would be able to take advantage of the better mechanical properties of a vacuum bagged lay-up.
The key here would be to do tests with sample pieces using identical core materials and adjust glassing schedules with the bagging to see if there really were any savings of weight. Either that or you could just go for it and do whole boards to see how it would affect the overall weight and ride characteristics.
Personally I’ve been doing hand lay-ups lately with the epoxy because it’s a quicker, cheaper, and a faster way to glass a board. To save weight I do a cheater coat just for sanding and then shoot the board with an automotive clear. Looks like a poly gloss coat the only bummer is that the isocyanates are lethal.
However I’ve been thinking of a project parabolic eps blank, carbon foot patches, 1/8" corecell deck flex plate with kevlar and S cloth bagged and painted with some pimped out gold metal flake paint.
You might be looking for your lost keys only where there is good light 
most things being equal, the post cured laminate will likely be more durable
Id also say the thickest heaviest laminate (hand wet) but the reality is theres too many variables to ask such a simple question. Composites are about strength to weight …take out the lightweight requirement and the whole game changes.
Good thread, thanks. Nice to hear what the guys with real experience have to say. No real vacuuming experience here but I’ve been keeping my ears open and want to get the process down. I ran across a commercial kayak builder who had an aerospace composites engineer compliment him on his very good wet hand lay up results, done one ply at a time, the engineer told him he wouldn’t get any practical improvement with vacuuming. I still want to get it figured out, for micro bubble extraction? , for positively clamping the laminate tight onto the form?, for smashing the laminate as thin as possible so it doesn’t expand, sponge like, to want more resin. John C.