Good points from all. I've never "filled" the fiberglass weave so I don't have any experience with that. Maybe it is just me, but the 0.73 oz cloth has a pretty small weave, hardly noticeable by eye (maybe I'm going blind). So the answer is, no, I do not use fill coats. Every coat of anything adds weight... even paint.
It is true that if you want a car-like finish, filling and sanding is in order. On this race model, I'm trying to keep it as light as possible because the wings are so small. I'm not making the 1/4 scale any lighter by adding retracts either. I hope it flies...period.
Adding fiberglass does provide more strength. If you peel the paper, overlay with 0.73 oz cloth and squeegee the epoxy off really well, the foam sandwich comes in about 10% lighter than with the paper. It has its pros and cons. I like the way fiberglass stiffens the foam. Having said that, fiberglass will not prevent damage in a crash, it kind of rips apart whereas paper will fold and bend - the same outcome in a crash is plane destroyed. However, the true benefit of fiberglass is that it is waterproof, will not warp and does a great job of preventing hanger rash. If you want you models to last longer than _____________ (fill in the blank like one dewy day, a week in a humid hanger, sudden rain) then fiberglass skins are the real deal. I've actually used the water hose to wash mine... Can't do that with paper!
I am curious for those who glass the DT foam if they do multiple coats on both sides until the weave is filled, or one coat on both sides, or one coat on pieces that will be on the interior of the craft, and then enough coats to fill exterior pieces.
Sorry if I don't understand the question... are you referring to coats as layers of fiberglass or overcoats to fill weave? If you are asking about coats to fill the weave, others who have done that will have to answer. I've never put more than one layer of fiberglass onto the foam unless I am adding structural support. Most of the panels in the planes I've built are single layer of fiberglass on both sides of the foam with no filling overcoat. Over this past summer I made a super light wing with one layer of 0.73 oz cloth on the outside surface of the foam. It was really light and flew like lightning!
Weighs 14.2 oz.
Looks like this now:
Lost elevon control in a steep dive going full throttle... spiral of death was really exciting!