The Dexter Hysol used to bond the spar caps etc has a short cure time as explained in the plans. It is best to keep the shop temp on the cool side but I wouldn't go below about 70 degrees. If you do, the stuff will be way too thick to work. It is VERY thhhhhick and hard to stir as explained in the instructions. You really need the extra helpers and you need to have it all planned out.

Be sure to pre-measure the Dexter Hysol into individual batches of separate resin and matching hardener before starting any mixing. If you pre-measure more than you need, you can always return it to the can, as long as you haven't added the hardener of course. It is so thick and hard to work with before the thin hardner is added that it takes awhile to measure out the individual batches and that time can be a problem for the person trying to do the mixing.  He just won't be able to keep up with the people doing the application if he has to try to measure each individual batch as he goes. If the resin is already sitting there in numerous individual mixing cups and the hardener is already pre-measured for each cup, then all he has to do is pour the hardener into the resin and start stirring.

At first it won't seem like the resin is mixing with the resin at all! But then as it begins to be assimilated, the stirring gets easier. Excessive stirring speeds up the curing process, so don't over do it on the stirring. As soon as it is thinish and easy to stir, stop the stirring. Don't allow the mixing person to get too far ahead of the applicators. As the Dexter is sitting in the cup, it is a relatively large mass. The heat generated by the catalytic reaction of the hardener tends to be retained toward the center of the mass (cup) and tends to bleed off from the outside surfaces of the cup. The heat collecting in the center of the mass of the material builds on itself because it has no place to escape. It is self insulated and self propagating.

Heat increases the reaction time of catalytic reactions. Therefore the hotter a catalytic reaction becomes, the faster it becomes hotter! If the material is allowed to sit for very long without being disturbed, the heat in the center can literally reach the point of "exotherm" or "runaway reaction" and can start to smoke and even burst into flames and possibly explode. Although I have never seen that happen I have seen some pretty violent reactions that were kind of frightening!

Therefore, if you have a cup of material that has been mixed for several minutes without being used, it is wise to do just a quick stir in order to distribute the heat evenly throughout the cup and prevent it from building in the center. Stir from the center out. Again, DO NOT OVER STIR. Stirring also increases the speed of the reaction. There is a sort of a balance between stirring enough to keep the mix as cool as possible, and stirring as little as possible to avoid speeding the reaction any more than necessary.

Of course, the best thing is to use it immediately when ready. A good rule of thumb is for the mixer guy to start mixing a new cup as soon as the applicators start applying the first cup. The applicators split (share) each cup to get it on as fast as possible. The mixer can delay starting the third cup a little if the timing on the first two was a little too quick, etc., etc.. If the mixer has a cup mixed before the applicators are finished with the previous cup, one of the applicators can start applying the new cup while the other finishes the previous cup. The point is to keep the process moving in whatever manner works and adjust as necessary as you proceed. Avoid panic. The adjustments will be small, but do keep an eye on the clock and gauge the progress accordingly. If you get 15 minutes into the process and you only have 1/4 of the process accomplished, you better get into high gear!

Once the material is applied to the surface of the spar caps, it will be spread out in a thin layer. If the glue spreader (with teeth) is used as instructed, the Dexter will lay on the surface in small parallel rows. There will be very little thickness or "mass" compared to the material in the cup. It will also have a lot of surface area from which any heat generated by the reaction can dissipate. At that point, the curing action will not accelerate on it's own, but will progress in a much more linear manner. There will be no danger of exotherm at all. However, the material still begins to cure seriously in about 30 minutes and the skin should be in place and being weighted down before the first batch mixed reaches that "age".

Applying clecos and weights
The clecos should definitely be in place BEFORE the weight is applied so the skin can "home" properly into position. You need "SERIOUS" weight to apply to the skin after it has been located. You can put some 1 x 4's or strips of plywood or whatever on the surface of the skin to spread the weight and prevent bruising the surface of the skin. Of course the weight should be located exactly on the spar area. Not to one side or the other. Car batteries, concrete blocks, 5 gallon buckets of water, and such things can be used. There should be very little space between the weights and there should be NO weight to the sides of the spar area which would cause the skin to "bend" or flex over the spar and deform the shape.

I point a Hair dryer into the end of the wing after everything is set. This warms up the Dexter as much as possible. The initial warmth causes the Dexter to soften and squish out as much as possible, and after that, any additional heat serves to speed up the cure. If you put a short length of 1 1/2" PVC tubing into the wing and blow the hot air into it, it will deliver the hot air down to the full depth you can reach. That takes care of everything outboard of the end rib. Inboard of that, about all you can do is put heaters under the wing so the rising heat warms the whole thing and work a couple of hair dryers over the surface on top. Of course this doesn't do a lot of good because the foam core insulates the heat from the interior of the wing where the Dexter is, and all of the weights get in the way of getting the heat where it really needs to be?

I have always found that the most important thing is to get the application of the material done as quickly as possible and get the skin on and weighted down as quickly as possible. When you peer through the cleco holes, even the material in the center area should be obviously squeezing out even though it is not getting any particular supplemental heat. 

When inspecting for "squeezeout"-
Be aware that the flanges of the spar which are going to bond to the carbon fiber cap are slightly flexible. If you apply pressure to the edges of them between the ribs, they will flex. When the skin is placed on the spar to which the Dexter has been applied, the skin will be pressed down by the weights until the solidarity of the actual spar in the center is reached. This will result in the Dexter in that area being squeezed out toward the outer edges of the flanges. As the Dexter is squeezed out, it will naturally build in volume as it moves toward the edges. Due to the viscosity of the material, this will have the effect of pushing the edges of the flanges slightly downward. The flanges in turn will push back against the Dexter harder the further they are forced down. After a few minutes, the process will reach a point of averaging out (balance) to where the Dexter is thick enough that the flexing flanges can't push any more of it out of the way. If the instructions have been followed, it is at this point that there should be some material being pushed out of the bonding area and some degree of material should be exposed and or dripping down onto the interior surface of the bottom skin.

All of this is good to consider and I mention it because it is very important to understand everything you can about the process BEFORE you start. It is easy to decide during the process that something would be a good thing to do and then waste a bunch of time on that effort only to realize that it was not practical for some unanticipated reason. By that time, it may be too late and the whole job may be botched? I have already made most of the mistakes and by sharing these observations, maybe you can avoid them. At least I hope so.

Tom