How best to fill them? I Made some garage doors last year and they were
faced with pine t&g floor boards. over summer, gaps of up to 5mm have
appeared which I need to fill and repaint over. I have just tried some
premixed Polly filla and some decorators filler(in a trigger gun) that I
have around, but the pollyfilla goes to hard to quickly and the decorators
filler doesn't give a good finish.

am I best buying and mixing my own filler or is there any better products to
use?

Steve