hi mico ballons will weaken a body, when you pour and the foam rises blok your pour whole and vent. that will make the foam to collapse the bubbles and create a very hard outer shell.as for paints that will not affect a foam body.we use a 15lb ridid and never have your issue.
we are in canada. box stores sell it in many forms.it cutseasily,sands easily.it holds screws better then pine,s .yes some folks have sensivity..the best cedars we use is white cedar from the east coast.