Not difficult - it is pretty much solid at room temeprature (but you could squish it in your hand easily). If you heat it up it gets real runny and then is easily applied inside sills etc with a sprayer. The ones you buy are basically like a trigger sprayer with a tube on with holes in it. It would be easy enough to make an applicator.
Only thing is - that link you posted seems to be charging over $200 for a 5L can - over here you can get a 5L can for less than $50. That seems excessive to me
