The way I do covering is to tack down the edges of the piece with an iron, poke a few pin holes to let out air (this is on a solid foam wing, with a built up wing you dont really need the pinholes) then shrink the covering with a heat gun 3-5 times (let it cool between shrinking), on the last shrinking I press it down with a glove/cloth working any bubbles to the pinholes(careful it will be hot) this results in a smooth finish that will not need additional shrinking through the year.