I use a retinol body cream at night and a body cream with SPF during the day.
First Aid Beauty Bump Eraser has helped my keratosis pilaris and bacne (and scarring from both).
It is sun damage and sadly I have crepey arms too. 😥 I started using this product in the last year. I think it has improved, but it is still there. I now wear sunblock on my arms and/or long sleeves to protect my arms from the sun.