On Tue, Aug 31, 2010 at 11:43 PM, Dennis Cox <dec720@att.net> wrote:
Numerous rules of thumb. Best summed as as paint it white if it is a room curing system and you don't want any print through. For post cured systems, it will usually handle whatever temperature you post cured it at. 90C is actually high for most timber boat building epoxies which are between 50 and 65. There are still plenty of boats around which were built using 45C systems back in the early days. And some of these are painted black. I have never heard of a boat structural failure related to exceeding the hdt.
rob
That's pretty much what I was assuming before I read that line of theirs. But its glad to hear it from you. I haven't seen anything on their site about the glass transition temperature. I know they say their epoxy is only made to handle up to 195F (90C) in use. Is there a rule of thumb?Dennis
Numerous rules of thumb. Best summed as as paint it white if it is a room curing system and you don't want any print through. For post cured systems, it will usually handle whatever temperature you post cured it at. 90C is actually high for most timber boat building epoxies which are between 50 and 65. There are still plenty of boats around which were built using 45C systems back in the early days. And some of these are painted black. I have never heard of a boat structural failure related to exceeding the hdt.
rob
From: Rob Denney <harryproa@gmail.com>
To: harryproa@yahoogroups.com.au
Sent: Tue, August 31, 2010 9:09:37 AM
Subject: Re: [harryproa] Epoxy Usage
Finally... the question...What happens if you apply a fresh layer before the set time has completed?Thanks,DennisNothing, although there may be a weak point at the join between the partly cured (not dried, it is a chemical reaction, not evaporative) tow and the new application, but for one tow this is nothing.I have never seen an epoxy where a second layer cannot be applied over a partially cured first layer so I suggest checking that statement with the supplier.Generally, the longer you leave it, the less likelihood there is of a chemical bond. The other thing to watch for is bloom, which is a waxy surface which develops on epoxies, especially in cool humid conditions. It needs to be washed off (hot water and a scotchbrite pad) before the next layer is applied. For this reason, it is best to apply wet on wet or at a pinch wet on green (soft but not sticky). For laminating, this should not be a problem. For bogging over a laminate, it can be, so I usually bog the laminate in the same session as the glassing.Most epoxies benefit from a post cure, but a thick laminate should develop a fair bit of heat, which will speed the cure time. Every 10C increase halves the cure/post cure time. Be careful post curing wooden structures, they warp.Anything you are worried about, do a little test sample. Epoxy instructions tend to be for worst case scenarios. The stuff is incredibly tenacious and pretty difficult (not impossible!) to stuff up. A test piece usually removes any qualms you may have. In this case, I would coat a piece of timber, leave it for two hours, then apply some glass. When fully cured (put it in the oven at the glass transition temperature of the epoxy for a couple of hours when the wife is out of the house) try to peel the glass off.rob
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