I always use a 2 part epoxy filler. Originally developped for the automotive sector it is now used everywhere. I have a convenient dispenser for it so I can buy large but cheap cannisters. €15 for 1,5 Kg. You can find it everywhere in the world in many different packages, like tubes or cans, but the smaller packages tend to be more expensive.
You just mix the two components together, apply it, and after 15 minutes it is hard as a rock. So work fast. If you want it to dry even faster, add more of the harder component, and if you want it to dry slower, add less.
I really dislike 1 component fillers, they're slow to work with and tend to deteriorate very fast. They also shrink when they dry, so it consumes a lot more time to get your work flush.
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