Recently I picked up a frog skin helmet cover that was absolutely caked in rust. It didn’t come with a helmet. I was considering removing the rust but I don’t want to hurt its value. Also I don’t know the best way of going about it without also damaging the fabric and fading the camo.

Lemon juice I understand removes rust stains. 

Whink makes a good rust stain removal product. I have had good luck with it on occasion, but I am unsure of its long term effects because it is an acid base. Do not let it sit long on the canvas, that is certain. I am also not sure how it will react to varied colors such as this. From a rust stand point it would remove most of this.

do a simple search online for rust removal thats recommended for textiles or textile conservation methods used by museums and conservators. First I would remove any of the rust or as much as possible by brushing with a soft bristle brush or even lightly vacuum with a brush attachment

"... brushing with a soft bristle brush or even lightly vacuum with a brush attachment"

 

> Yes. And, another useful tip - when vacuuming textiles a neat trick is to place a piece of ordinary window screen on the fabric and vacuum through it, which keeps from damaging the object and allows for more suction.