There are three ways you use epoxy in cold-mold boatbuilding:
1. Gluing: When gluing, you typically want to mix in enough Silica to give the epoxy the consistency of creamy peanut butter. At the very least, yogurt. Epoxy is a gap filling adhesive. If you are more familiar working with more "traditional" types of woodworking adhesives such as Titebondtm, then there are a few differences you must be aware of.
One difference deserves special attention. Be very careful that you do not overtighten clamps when gluing with epoxy. Wood glues like Titebond require a great deal of clamping pressure to work effectively. If you try the same technique with epoxy, you will literally squeeze out all your glue and create a starved and weak joint.
The epoxy is ALWAYS mixed at the ratio of 2:1. Two parts resin with one part hardener. You never try to speed up or slow down the "kick time" by varying the amount of hardener. This is a common practice with polyester resins. Don't do it with epoxy resins.
To vary the working time of epoxy, you must use different formulations of hardener. They are usually described as "Fast" and "Slow". Imagine that... We have historically supplied only slow hardener to our builders. This gives relatively inexperienced builders the most working time possible. However, "Fast" is available and we are considering recommending that future builder buy one gallon of "Fast" for those times when gluing something small and you don't want to wait. The "kick" time with our resin can be varied by simply combining "Slow" and "Fast" in whatever quantities give you the working time you desire. This does not change the fact that you MUST keep the resin to hardener ratio at 2:1.
There is one "approved" way of varying the working time while using only one type of hardener and still keep with the 2:1 ratio. That is to vary the temperature. Warmer = Faster (and of course, Colder = Slower). The curing process of epoxy involves an Exothermic chemical reation, meaning it gives off heat. If you contain this heat, by using your mixed resin out of a deeper container, then the resin will "kick" faster. Pouring into a shallow container will give you more time. Working with thickened epoxy in direct sunlight will clearly reduce your working time for two reasons. Obviously the sun's radiant heat on the objects you are trying to glue is one factor but the heat absorbed by the thickening agent itself is another factor.
2. Fiberglassing: There is not much to say here about using our epoxy when fiberglassing other than "Don't thicken it!". The same concepts apply regarding "kick times" The actual process of fiberglassing will be covered in the Methods section of the forum. We often get the question: "Do I need to sand between layers when I fiberglass." This is purely a function of time. If it has been more than 24 hours between layers, or if anything such as dust, has contaminated the surface, then the answer is "Yes!". This is a judgement call. If in doubt at all, lightly sand.
2. Hotcoating: This is really a slang term. When used in this context, it means to use resin alone - without any binder - such as fiberglass - to seal wood. An examples of where it is done in cold-mold construction would be the underside of a sole or deck. Application methods include brush, roller or spray. Normally it is a very bad idea to thin epoxy resin with a solvent in order to properly spray. Most, if not all, manufacturers strongly advise against it. Their product data sheets usually talk about the fact that it greatly reduces the strength of the resin. It is such a common practice in cold-mold construction for areas like the ones mentioned because the builder typically isn't concerned about strength. They are looking to merely waterproof the wood. There is one method to thin that is advised by the manufacturer and that is to warm the resin. This is difficult to achieve for two reasons. One. The resin will "kick" faster so you must move very quickly working in small batches. (Best with a helper) Two. Unless you have a gun with a very big orifice, this method of thinning will probably be insufficient. Denatured alcohol, in the most pure percentage you can find, is how most people thin when they feel it is necessary.