Certainly resistant, though may be pretty water proof, I myself was more interested in the insulating value for preheating in the hanger but made it outdoor grade for trips if need be, as being only a cowl cover even insulated it would fit in the cargo area... I made it from a jacket lining material facing the surface of the plane, almost a silky texture like the inside of a carhart coat.. Figured that wouldn't hold dirt to scratch, then 80 gram thinsulate insulation throughout, with a double layer on the top portion of the cowl (for me it was about R factor, but it could act as a "hail impact damper" and the shell is made from an outdoor nylon material- much like a backpack is made of.... That outdoor nylon has to minimally be water resistant... I would assume even it it was water proof the stitching lines would allow some water through if exposed to torrential downpour, though I think there is a product one can put on seams to prevent that... If one goes for it, you probably could find a different "Insulation" material, as I chose thinsulate for its high heat retention to thickness ratio. You could probably get a "lower quality" insulation that was just puffy for padding-as a whole plane cover you wouldn't need to worry about thin but high R value. Being a lot more cover than mine so likely not taking it in the plane the bulk wouldn't be of as much concern.
The insulated cowl cover & insulated prop/spinner cover from Bruce's would have been $600 before shipping, I spent about $130 on material. With the time investment Bruce's prices are fair by all means!!! But for me I found enjoyment on nasty winter evenings I couldn't fly anyway working on it... Wing covers would be pretty simple- just a monstrous pillow case basically...
If you are thinking about it, the other thing you want to do is buy an OLD sewing machine... Pre 60s... I learned that from boat seats, that if you aren't going to invest in a true commercial sewing machine (which would eat all the savings plus) you cant do all this thick material on these cheapies you buy at walmart. I found the perfect one- an old singer, literally from between 1910-1920! Owners manual brags about the "Ease of sewing with electricity" ! Many like this available at every thrift store and estate sales (Mine was $20), but its build like a brick poop house, all solid metal so it won't break plugging through the thick material like plastic parts in a new one would. The other advantage to those of us that sewing is not our first skill set is it still has the inertia wheel on the end, so for corners or difficult spots I could spin in by hand, stitch by stitch, so I could go slow to get it right...
