I have an optrel helmet - I never get that trouble but I do with almost every other type of safety mask or goggles.
For anything other than welding I use an Avon S10 gas mask. Never mists up and rather good protection when sanding
In a pinch you can use your own saliva. Just rub a small amount over the lens with your finger and gently wipe it off. The residue left behind should prevent fogging. This is what we do when snorkeling or scuba diving to prevent the mask fogging up. You can buy commercial products, but in the end just a bit of saliva works fine; try it ?