“Normal” BONDO (here in US) is a putty of polyester base. Cougar uses stuff that has short “hairs” of fiberglass mixed in to give extra strength.
I usually put a dab about the size of a small spoon on a plastic dinner plate. Then I splash on a few drops of acetone and mix in until it is of thin consistency - maybe like sour cream for bagels (???) Once I get consistency, I add in the red colored hardener and stir to have an even reddish/pink color. I use an old credit card or polyethelene plastic scrap and quickly spread the filler on as fast and as complete as possible. If you don’t work fast, this small bit of putty/filler will become hard and unworkable in less than 2-3 minutes. Mix only what you can spread quickly. Too much hardener ( drops) and it will “go off” even faster.
Don’t worry if you miss any area, or if you has ridges from the end of the plastic card. You can add a second coat for missed areas, and sand off any high ridges. If you build up really thick, you can use a cheese-grater to quickly fair it down. On flatter surfaces I use a straight edge of broken window glass as a scraper. You will quickly find the right amount of time for hardening, and when the plastic filler can be worked. If you let harden overnight ( or maybe 5+ hours) it will be easier to sand off while if still “green” scraping will work faster. It’s a dusty process so may want to work outside.
While at the auto store, ask them for “SPOT” body filler - usually a small tube about twice the size of toothpaste tube. This is red or green (no matter - just easier to see) and is very fine filler. Good for pinholes or surface imperfections. This stuff really gets hard fast, so I use just a piece of cardboard from a soda case to spread/dab on quickly. This stuff sands even better than the BONDO, and is made to allow feathering in edges of the filler to surrounding surfaces. I think BONDO als makes this stuff too.
This material will ssoak up water/moisture, so be sure to prime and pain before floating the hull. Follow Caludio’s paint plan to find low or high spots.
:graduate: Ha - bet you didn’t realize you would become a somewhat experienced automotive body worker when you decided to try this hobby - did you? 
Might want to practice a bit on a scrap piece of lumber so you know the set-up and hardening times for the body filler. Wish you lived closer, I’d walk down to your house and talk you through the process. Text and description makes it sound harder than it is. Just dive in and pretend you’ve done it before, is best advice I can give. Soon you will look back on all you have learned and be able to pass it on to local sailing friends.
Hang in there - don’t get discouraged.
Dick
PS - once the BONDO gets hard, it will pop off anything of smooth plastic like you spreader cards, mixing plate, etc. Not so much from aluminum sheet, and definitely not from paper or cardboard. While at the auto store, also pick up a small packet of flexible plastic spreaders. Here they cost about $3.00 - and if you have an auto body shop close by, perhaps they will let you watch as they use the body filler - or may even give you a free lesson. Just take your hull plug or finished boat back to show them. They will feel they were responsible by helping you to build your hull.
. Could I just use good quality wood filler on base balsa, then use fairing compound on fibreglass? Or is this up to builder preference?
. Jolly good. Funny, I have Poms and Yanks giving advice to an Aussie about products which may not exist here :rolleyes:.