Thankyou for your question! I am little troubled to here that your Supplier told you that Fass-Dri only works when the sun hits it??? Well the sun does play a part in the drying process, but Fass Dri was developed to dry the sealer in various ambient lower temperatures and night sealing. I have hundreds of contractors that use Fass Dri PSA in their sealer while sealing at night with great results. Fass Dri psa utilizes Nano cure technology that dries thru Nano friction that causes molecular heat. It dries the sealer from the bottom up! In colder temps the additive may take a bit longer to kick the drying process, but will perform well in cooler temps. There are different factors also that affect the performance of additives, mix designs play a huge roll in the drying process as well, if you are adding over the manufactures recommendations for water, you may retard the drying process some, also if you add the second coat over a first coat while wet or damp may cause the sealer to dry a bit slower. But adding Fass Dri psa to your sealer gives you the comfort knowing that your sealer will not only dry faster, but help the curing process and sand suspension that only Fass Dri psa gives you. Ask you supplier to please call me at 800-892-6701 and i will discuss his reasoning for the statement he made to you. It is simply not accurate on the Fass- Dri psa, it works with great results as long as rain is not imminent while wet… I have contractors calling me all the time ranting and raving on how Fass Dri psa has extended their fall sealing season as much as a month just by adding Fass Dri Psa. I hope you will try Fass Dri psa in your mix and see for yourself the beauty of fass dri quality as it was designed, to perform. Thanks!