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Old 05-09-2007
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Question Polyester Resin Question

Ive got a 36' sailboat with numerous screw holes in the deck from 30 years of hardware being installed and removed. I'm going to start patching them tomorrow with polyester resin. Should I add an adhesive filler to the resin when patching small screw holes or just use the resin straight?
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I'll give you an unequivocal answer...: It depends

If the hole has a bigger diameter than, say 1/8", it might be worth thickening up the mixture, but more important, i'd have thought is to keep it relatively runny and smoosh it down into each hole with the end of a matchstick, so you can ensure that all of the holes are fully filled.


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Old 05-09-2007
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I wouldn't use polyester resin. I would use thickened epoxy instead. Polyester resin isn't as waterproof as epoxy resin is, and its ability to bond to old resin is much weaker than that of epoxy, so it is very unlikely to give you as good a waterproof fix as epoxy would. Polyester resin is also significantly weaker in tensile strength than is epoxy resin, so not as suitable IMHO for repair work.

Also, I would recommend drilling the holes out a little, since it is very likely that the person who drilled them filled them with some sort of sealant compound, like 3m 4200 or such...and the resin (epoxy or polyester) really needs a clean, dry, firm surface to attach to.

I think both of these steps are far more important on a cored-deck than on a plain fiberglass deck, but would do it for either.

Finally, if you're set on using polyester resin, you'll probably want to wipe down the screw holes with styrene, to try and "re-activate" the existing resin and give the new polyester resin a chance to adhere better. However, styrene is fairly toxic... so you really need to wear gloves and a VOC respirator while using it.

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Styrene is dangerous, and a waste of time. All bonds of polyester to polyester will be mechanical (and epoxy to polyester for that matter), so surface prep is key: sandpaper and lots of it. There's nothing wrong with using polyester for a repair (it thickens nicely with colloidal silica, just like epoxy) especially large repairs, since it's 1/3 the cost of epoxy. For filling screwholes and the like, it's perfectly suitable, particularly if you plan to repair the gelcoat afterwards. If you're planning on applying gelcoat, you CANNOT use epoxy as the gel coat won't stick to it. The added strength of epoxy is essentially useless in a screw hole, and the cost makes it not worth it. Save the epoxy for structural work.

For your screw holes, you need to oversize the hole by 2X - 3X. Fill with unthickened resin, then drain from the interior or draw out the excess with a syringe. This ensures saturation of the old material. Then add your thickened resin to fill and close the hole. Oversizing the hole ensures that when you replace the hardware, you'll have resin all around the fastener, not core material.

Best advice: Go buy one of Don Casey's books. All these questions and more are answered clearly and concisely.
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