Originally Posted by ECONORAM
Agreed! In fact, this gives me an idea... I tested a near full grill block on my truck and found a 1% FE decrease. However, an idea I've bounced around is to fab a small Daytona nose shape (1969, to be exact) to the front of my truck. Or maybe a bird beak shaped nose in front of the grill. I figure the sloped/beaked nose would give better FE results. -->Never thought of using gap filler foam. Might have to try that.
For something that large, they sell expanding foam in a two-part setup. Base + activator I believe. Build a form, mix it up, and pour in.
Also, the "max-fill" product that I used left some considerable sized air pockets inside, which made the wood filler essential (and a PITA). The formula they sell for doors and windows expands minimally, and is denser, which makes for a highly sandable/workable product compared to the super expanding stuff. I would imagine that just a quick skim coat would be necessary if you were topping over that. Although you could use the max-fill to fill the large voids, shape it to a little smaller than you need, and then top-coat it with the dense foam for the finish.
The wood filler also seems to be rather brittle. Just from the commute today, I have a few cracks that have developed right in the middle under the logo. I think once I have the final sanding done, I'll top it all off with a coat of epoxy just to add to the rigidity.