Depending on where the mold is and how deeply it has penetrated the fabric, it should be possible to remove the mold/mildew without damaging the dry bags.
– Don’t use very hot water. Water above 60°C / 140°F can damage the seam tape. – Don’t use bleach. This can damage the polyurethane seam tape and the coating. – Don’t use solvents (including dry cleaning); they can cause the coating to delaminate.
Start by hand washing the dry bag in moderately hot water (around 45°C / 115°F) and a non-detergent soap such as Sea to Summit Wilderness Wash. A sponge should work well for this; if you use a scouring sponge, be very gentle with the abrasive side.
If the mold is too deeply ingrained to be removed by the washing process described above, you will need to use a product from McNett called MiraZyme (an enzyme based cleanser – details at http://www.mcnett.com/MiraZyme-Enzyme-Based-Odor-Eliminator-P191.aspx). Apply a diluted solution of MiraZyme directly to the dry bag using a sponge – this should kill the mold, although it will not remove the discoloration.
Alternately, you might try salt (1 cup / 250ml) and lemon juice concentrate (1 cup / 250ml) in hot water (1 gallon / 4 liters); again, use a sponge to apply the solution and avoid scrubbing too hard. Rinse very thoroughly after applying this solution.
After any of the washing/rinsing procedures described above, dry the bags in the sun until you are confident that they are completely dry. Do not put the dry bags in a dryer; the heat can damage the seam tape.
If the dry bags have actually been damaged by mold (ie the mold has degraded the coating or caused the seam tape to delaminate), there is no way to restore the bag to its full waterproof performance.