I have relatively sensitive skin. I have an osprey waypoint 65 backpack.
The shoulder straps chafe my shoulders, especially if I go without a shirt.
Is there a softer fabric I can attach to the straps or other solution to prevent this?
I recently had this problem with a pack and have experimented by sewing on a layer of velour. It's much softer and padded. However it is also VERY warm, which may defeat the purpose of going shirtless.
Chafing occurs when skin rubs against something whether other skin, clothing, or gear. Staying clean, dry, and reducing friction are the ways to prevent any kind of chafing.
As Ben said above that your hip belt may need be used the way it should be. I struggled with heavy packs, so I started following a way to adjust my pack this way: Loosen all the straps, Adjust the hip belt, over the hip bones. If the belt sits too high, it might constrict the stomach. If the position is too low, the fins might chafe. Now tighten the shoulder straps, but not too tight. The main weight should be on the hip belt. Once you have found the perfect back length, fix and close the height adjustable sternum strap. Not too tight, since it could hinder breathing. The strap stabilizes the shoulder straps and is a standard feature of almost all packs.
If you wear a shirt/T-shirt that doesn't fit neat, it may cause chafing after a long walk with a heavy backpack. Wear synthetic fabrics. Clothing that wicks moisture away from the skin significantly reduces chafing. When you are without a shirt, then probably some anti-friction lotions might help, but with a heavy pack I'd never go without a shirt.
IMHO, I am rather a kick-the-root-cause kind of a guy, and this lotion thing sounds like a work-around solution which I don't like. Rather than going for such products, I'd prefer to add an extra foam cushion under the shoulder straps if the problem persists.
EDIT: Looking at the picture of the backpack that @liam has added, I don't think that pack would trouble me and most of other people, as IMHO, the shoulder straps looks good with cushion. But thats just by the looks of it. I'll just say, don't invest in for a different backpack for that reason too soon. Its a good backpack.