As I stated in a previous comment,
We had our tires balanced with Equal balance powder on our 35DSRL Eagle Ridge.
"This is suppose to replace the traditional clamp-on weights used to balance tires. The unique thing about this product is that unlike the clamp-ons, this product is blown into your tires via the valve stem through the use of the Equalizer machine, found in 4Wheel Parts or other authorized Equal [COLOR=#ff4400! important][COLOR=#ff4400! important]dealers[/color][/color]. The product moves around inside the tire to correct balance infinitely, while your tires are rolling on the road. The manufacturer also claims that you won't ever have to re-balance your tires again.
Balancing your tires with EQUAL compensates for irregularities found in both the tire and wheel assembly and at the tire road surface contact. EQUAL has the ability to adapt, respond and dampen vibrations at the tire footprint."
and, NO, moisture will not degrade the powder. My Brother-in law who works for Goodyear has seen hundreds of tires that have used this product, especially semi trucks. Best time to "throw in the bag" is when you get your tires mounted, but we just had it applied to the existing mounted tires. Really simple install. here's a link to the product:
http://www.imiproducts.com/equal/faqs.aspx
PS be sure they change out the valve stems with the special EB ones as they have a check valve to preclude the loss of powder upon inflation.