qldviking

Ok, fillers come down to personal choice and whats available to you. I'd go with balsarite, its light and easy to sand etc, many fillers are hard and its so easy to sand a skin next the filler away to nothing. if you want to seal and fill and then paint, I'd suggest another alternative, polyesther resin with microfibres or balloons, mix it thin and spread evenly. i find old credits cards work very well for applying, best results given working diagonal to the grain. Why I like polyesther over epoxy is it works well thinned down and with retarders to give you working time, and still paints well.

The Cracking issue is in reality quite simple in its causes, All the balsa is only sealed from the outside, the inside is still quite raw and quite happily still steadily drying out some more over the years, even in storage. What happens is the balsa absorbs moisture during humnid spells and then losees it again during low humidity, the movement isn't a lot but it slowly draws all the resins and oils out of the balsa's grain which is why it slowly shrinks and cracks. Its these oils and resins which give the balsa {and any timber} its structural integrity.

I am a cabinetmaker/joiner/shipwright, and over the years have seen mny change as to filling sealing and coating woodwork. My personal preference for coatings and fillers is the urethane family, they give the best long lasting results in harsh enviroments.