Quote:
Originally Posted by Locke N Load
If I understand it correctly, strength will be the same between 1708 and 1700 layer to layer, but a buildup with 1708 will be stronger because you get a little bit of the corrugated box effect with the spacing from the CSM in multiple layers. Is this accurate?
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Build up with 1700 of the same thickness will be much stronger than 1708 of the same thickness. This is due to the resin to cloth ratio. The long strands of 17oz biax is much stronger than the short cms strands. More layers of tightly spaced strands of cloth and less resin makes a stronger lighter lay up.
Now saying 3 layers of 1708 vs 3 layers of 1700 (layer vs layer) the 1708 would have more fibers with the extra cms and it would be thicker and build up faster. So the thicker 1708 with the extra resin and cloth would be stronger in that instance. But if you did 5 layers of 1700 vs 3 of 1708 it may use the same amount of resin and be the same thickness and the 1700 would be much stronger. Not sure if the 5=3 here but just for the general idea.
The box effect you mention would add strength from extra resin spacing with the csm but for me I want strength from the layers of long biax strands which are much stronger than short random cms strands. Cms just takes too much resin doing a hand lay up.
The only time I would use 1708 with epoxy is if it was all I had at the time or if the top layer needed fairing. Then I would place the CMS layer upwards so once sanded it would cut into the csm and not break the 17oz long strands. But like BigShrimpin mentioned trying to sand gummy epoxy is not ideal. In general sanding epoxy for any reason is not easily done.
I like the idea of using all types of resin Poly, VE and epoxy. The Epoxy is a must for strong secondary bonds to old glass. The VE is great for decks and parts that are wet on wet laid up. Poly is an economical choice for parts popped from molds with gel coat first layers. So each has its place. Doing a boat 100% epoxy is great if you can afford it but the other products work almost as well if used correctly in the right way. Look at the 60-70's vintage SeaCraft hulls that were all Poly. They have some signs of water absorption on the bottom maybe if left in the water a long time but they have held the test of time. Poly may not be as strong or you may not build up layers as light with it but it will do good if on a budget. If I had to chose only one it would be VE as it has the best of both worlds and a fair price but epoxy is hard to beat for gluing or bonding parts. I think if you can choose where to use what based on the parts needs you will save some funds plus to me VE and Poly are easier to use and the cure process goes much faster. It is easer sand and fair out over epoxy also so each has its benefits.
The main reasons I chose to use epoxy is:
1) super strong secondary bonds to old glass = no delamination
2) no water absorption like with poly = water proof almost
3) no csm needed so better glass to resin ratio = stronger & less weight
4) more elasticity so it flexes more before it cracks = flexibility
5) using high end fibers like S-glass, Carbon or Kevlar
If one of the above are not need I turn to VE. If I am doing a gel coated molded part I may use Poly.