epoxy vrs polyester

[blink2u]

Hi guys,I have spent days reading as much on this subject as possible ,but still am doing some head scratching .When prepping the underside of plywood for bulk heads,or flooring,and using epoxy(west sys),what is considered the procedure,will 1layer of resin rolled on be suffient,should the plywood be scuffed,do you need any cloth with the resin?
Now the polyester resin question,on the boat that I am working on( 32yrs old boat),all the tabbing on the bulkheads has been with 1 layer of woven roving,on each side,now ,if I wanted to do the same(in areas away from bilge water)and were prepping the plywood,a good scuffing in the area to be tabbed,then a thinned poly mix(now would 1 thinned mix be enough? to penetrate the plywood for better adehesion,before the heaver tabbing layer?Lastly,as with the question about the epoxy,can the thinned mix of poly be used to (encapsulate),plywood to protect from moisture ,that's away from the bilge and direct water emersion
Sorry for rehashing old territory,some of you experts get very techincal for a backyarder like me !Thanks for the help

[Stumble]

Blink,

Epoxy sticks to everything, poly is much more finicky. For retrofits, and repairs I will only use epoxy since that eliminates concerns of product incompatibility.

NEVER EVER EVER thin epoxy. It doesn't penetrate better, and it leaves a porous surface that is not water proof.

To encapsulate wood you typically need to roll on three layers of neat (unthickened) epoxy to ensure you didn't miss a spot, and that you have sufficient coverage. It is not enough though to just do one side, you really need to encapsulate the entire panel since this will also stop the woods natural expansion and contraction as humidity changes.

You do not need to sand, or scuff it. The epoxy-wood bond is stronger than the wood regardless of how it is prepped, so there is no reason to spend the time. It also created hollows or depressions that will be filled differently than the rest of the panel, and this could cause thickness issues over the panel.

Finally glass over an encapsulated board can provide a number of different things. A thin layer is great to provide abrasion resistance. A thicker layer adds stiffness and strength. It depends on what is being build and the required material properties needed that determines if a board need glass on top.

[PAR]

Blink, Stumble has got it about right. Polyester techniques don't work with epoxy and the opposite is also true. They are both plastic resin systems, but work chemically and physically different.

Simply put, polyester is used on production boats, because they are built to a price point. To this end, they don't really care if it's a good system or even durable, so long as the price point is met and they can crank out enough units, to pay for tool up and the production run, in the time frame they hope.

As Stumble mentioned, the best way (with either resin system) is encapsulation. The manufactures don't do this very well, as it requires more labor and materials in out of site locations and more importantly their methods, will easily last through the warranty period. Encapsulation is just how it sounds, entombing the wooden pieces with resin and sometimes reinforcing with fabrics. This said, polyester resin doesn't stick to wood very well, especially compared to epoxy. If you use either resin, you need three coats on every surface, particularly end grain, notches, screw holes, cutouts, etc.

As with any coating, you need to "prep" the surface, which with epoxy simply means sand it with 80 grit, clean the dust off and apply. With polyester, you need more "tooth" so use 40 grit, then coat.

Roving is a very coarse material and uses lots of resin and usually not the best choice. Directional fabrics are stronger and better for 'glass to resin ratios, when employed as tabbing. Polyester really can't take advantage of these fabrics, so bulking fabrics like mat and heavy roving are employed to help compensate. On all but the smallest boats, I spec 45/45 biax. You'll use half as much fabric and resin to get the same strength as the heavier stuff.

When you start playing with the chemical nature of these resin systems, you can easily run into trouble, without a good understanding of what you're doing. It would be best if you just selected a resin and stuck with it. Epoxy is the obvious choice on wood and with novice laminators, because of it's ease to get good, strong, waterproof results. If you want to use polyester, then you certainly can, but you've already seen how well this stuff works. Additionally, if you have to screw up, error on the side of too much tabbing, too much resin then not enough. The not enough side of the coin is why you're making these repairs, plus the physical attributes of polyester on wood. Had the manufacture used twice as much fabric and resin in the tabbing and bilge, you'd probably not have these issues.

[blink2u]

Thanks for the replys guys,I will take your input into consideration!