thinning resin

I am wondering if it makes any sense to thin the epoxy for thinner coats on the outside of the hull on the 17LT I am building. What are pro'sw and con's of more thinner coats and, if it makes sense what would be an appropriate thinner and how much?? Thanks. Just Dave

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RE: thinning resin

Dave - you will receive various opinions and points of view: here's mine!  


On my recently-completed Ches 17LT (and now being repeated on my under-construction Shearwater 17 S&G) I did use a thinner in the epoxy mix for the initial interior coats and on the hull and deck.   The stuff is called TPRDA (Timber Preservative and Reactive Diluent), and I'll quote from the label: "TPRDA is an epoxy based timber preservative which reduces viscocity of epoxy and enhances penetration into timber is used to give that all-important first penetrating coat to timber...add 1 part TPRDA to 5 parts epoxy, mix thoroughly".   It does appreciably thin the epoxy mix, and was recommend to me by my materials supplier.   I guess it might also be particularly useful if epoxying in a cold clime!  


I used it in the first 'priming' coat when either further un-thinned epoxy coats were to be applied with glass cloth - inside the cockpit and outside of hull: or when the epoxy was to be varnished over later - the decks.


It doesn't seem to effect the speed of curing in any way.


It is also useful to thin epoxy to function as a 'primer' prior to joining timbers, e.g for scarf joints (before the thickened epoxy is applied to make the joint): and applying to sheer clamps prior to putting the deck on with - again - an epoxy/ thickener compound: etc.   Again - it promotes penetration of the later epoxy mix.

At $8 per 250ml bottle it is not expensive, and probabably worthwhile: it can hardly hurt!   One bottle sufficed for me for the Ches.


Enjoy the rest of the build.



RE: thinning resin


Outer coats shouild never be thinned. After market additives reduce viscosity by adding volatile thinners which evaporate. These leave behind microscopic pinholes which allow the passage of water, totally defeating the purpose of epoxy encapsulation. Because of the pinholes, structural epoxy should never be thinned, either. This would be glue and, depending on the boat design, fillet putty mixes.

A better soultion is to buy low-viscosity epoxy, such as System 3 SilverTip, which has non-evaporationg viscosity agents as part of the formulation. These can be safely used on an outer coat, as well as for wetting out glass and making glue & fillet putties.

Good luck,



RE: thinning resin

Thanks guys. Excellent feedback. BTW - I tell my wife that kayak building is great therapy for the stresses in life ... as I retreat back to the basement to work on the 17LT. She wants me to build her one too. We will see how this one goes first. D

RE: thinning resin

The 2nd one is always easier, things make sense.


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