Generally speaking there are two kinds of fillers: one group adds structure to what otherwise be nothing but liquid glue, the other isn't as much structural as simply something to make that liquid glue easier to handle in use.

The chart shown here for WEST System products nicely shows which fillers should be chosen for a particular task. Understand too that you can use them in combinations to custom blend an epoxy mix for your preference of workability and structural integrity; it's not one over another. Those WEST fillers can be used with equal abandon with other brands of epoxies such as the MAS family available here & elsewhere.

As for 205 hardener for summer working?

Do you work in temps over 60F most of the time? 205 might be too fast if so, unless you work with very small batches at a time. Read up on what the manufacturer's recommendations are for hardener choices given different working and curing temperatures.

Apologies - seems that URL got copied/pasted wrong....

Here's a couple instead:

https://www.westsystem.com/the-105-system/product-selection-guide/

(Covers fillers & their uses, hardeners too)

https://www.westsystem.com/instruction-manuals/user-manual-product-guide/

(Multi-language manuals covering all kinds of things that can be done with epoxy)

ditto on spclarks note about really reading up on working times...particularly during the summer and especially when you are new to this.

there is nothing more frustrating then having epoxy kick (harden) before you are done with the particular task.   in the summer where i am at - mid-atlantic.....the shop is often approaching 90 degrees or more....so i exclusively use 'slow' formulations.

there are tricks to get the most time possible, as mentioned (e.g., the small batches) but that may provide little comfort when dealing with a larger segment like glassing a hull or deck....particularly the interiors which take longer than exteriors.   

actually recording your times when doing smaller steps is a good little habit to develop a strong benchmark of what it takes to do bigger steps.   

h

Fast hardener has its uses of course, why I chose to buy some from MAS when I was about to begin assembly of my Waterlust canoe kit last fall.

I was anticipating working conditions in my garage (insulated walls & ceiling, also sheetrocked & taped with a 50k BTU gas garage heater installed the year before) of minimum 50°F, bumped to 62 - 65°F for extended working sessions on an irregular basis. Turned out to be a good idea, otherwise I would have been waiting vastly longer intervals for stuff to cure hard enough to work with tools or sand. Much of what I assembled over the winter months was with fast hardener.

Then as I grew more proficient with mixing & placing the stuff I started using slow harder mixed with fast when I was expecting to need more time for assembly or I wanted to have more time to do stuff like remove excess with tools or use denattured alcohol on a gloved hand's fingers (both hands more than likely) to smooth fillets I'd placed. This is a technique I can't recommend learning how to do enough! Saves hours of sanding, particularly in places where sanding will challenge the most proficient.

When summer temps arrived early June the fast hardly ever got used. It was 100% slow.

Another comment: whatever hardener you use, adding fillers may shorten your pot life. (I think the fillers alter the heat-retaining characteristics of mixed epoxy resin & hardener.)

There were times when I was placing cell-o-fill thickened mix with plastic syringes when I could feel the plastic beginning to get warm as I was approaching the last of what I'd stuffed under the plungers. Same with microballoons for non-structural fillets and fairing.

So be careful when starting out with adding fillers so that you don't have more than you can get placed in a brief interval or you may get caught by surprise....

   For nearly all wood joints, epoxy needs a filler to build a good joint. Straight epoxy/hardener tends to run out of gaps and absorb into wood grain. To fill gaps, and there are always gaps with wood, you need the fibers or particles of a filler to build a matrix with the epoxy holding it all together. Wood flour is used for fillets because it makes a pretty strong matrix, is cheap, and about the right color to match the wood since its visible. Cabosil or the cello-fil from Mas is strong and smoother than wood flour so it works well in tight joints. I like the Mas products and always use the slow hardener cause I'm a slow worker. I've used West products and they are the only ones I've messed up and had cook off. I've actually had a batch fume and bubble like a volcano.